Ch-4(Road Design Elements)

TABLE OF CONTENTS

4

ROAD ROAD DE DESI SIGN GN ELEM ELEMEN ENTS TS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4-1 4.1 4.1

INTR INTROD ODUC UCTI TION ON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4-1

4.2

HORI HORIZO ZONT NTAL AL ALIG ALIGNM NMEN ENT T . . .. .. . .. .. .. . .. .. . .. .. . .. .. .. . .. .. . .. .. .. . .. . 4 4-1 -1 4.2.1 General General Controls Controls for horizon horizontal tal alignment alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2.2 Tangents angents.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3 4.2.3 Curves Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6 4.2.4 Superelevat Superelevation ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 4-8 4.2.5 Transition Transition curves. curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17 4.2.6 Lane widening. widening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18

4.3

VERT VERTIC ICAL AL ALIG ALIGNM NMEN ENT T. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4-2 -21 1 4.3.1 General General controls controls for vertical vertical alignment. alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 4.3.2 Grades Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-22 4.3.3 Curves Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-26

4.4

CROS CR OSSS-SE SECT CTIO IONS NS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 4-29 4.4.1 General General controls controls for cross-s cross-section ections s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-30 4-30 4.4.2 Basic Basic Lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-32 4.4.3 Auxiliary Auxiliary lanes. lanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-33 4.4.4 Kerbing Kerbing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39 4.4.5 Shoulders Shoulders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-40 4.4.6 Medians Medians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-43 4.4.7 Outer separato separators rs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45 4.4.8 Boulevards Boulevards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45 4.4.9 Bus stops stops and taxi taxi lay-byes. lay-byes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 4.4.10 Sidewalks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-48 4.4.11 Cycle paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 4.4.12 Slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 4.4.13 Verges Verges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-52 4.4.14 Clearance profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 4.4.15 Provision Provision for utilitie utilities s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 4.4.16 Drainage Drainage elements elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-55

LIST OF TABLES Table 4.1: Minimum radii for various values of emax (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 Table 4.2: Design domain for emax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 Table 4.3: Values of superelevation for above min radii of curvature (%): emax = 4 % . . . . . . . . . . . . . . . 4-11 4-11 Table 4.4: Values of superelevation for above min radii of curvature (%): emax = 6 % . . . . . . . . . . . . . . . 4-12 Table 4.5: Values of superelevation for above min radii of curvature (%): emax = 8 % . . . . . . . . . . . . . . . 4-13 Table 4.6: Values of superelevation for above minradii of curvature (%): emax = 10 %. . . . . . . . . . . . . . . 4-13 Table 4.7: Maximum relative gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14 Table 4.8: Lane adjustment factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Table 4.9: Maximum radii for use in spiral transition curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18 Table 4.10: Lengths of grade for 15 km/h speed reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.11: Maximum gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.12: Minimum values of k for crest curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.13: Minimum k-values for barrier sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.14: Minimum k-values for sag curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 Table 4.15: Warrant for climbing lanes. lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-35 Table 4.16: Shoulder widths for undivided rural roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42 Table 4.17: Warrants for pedestrian footways in rural areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 Table 4.18: Cycle lane widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 Table 4.19: Typical widths of roadside elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 Table 4.20: Scour velocities for various materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-57

LIST OF FIGURES

Figure 4.1: 4.1: Dynamics Dynamics of a vehicle vehicle on a curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 Figure 4.2: Methods of distributing of e and f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10 Figure 4.3: Attainment of superelevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Figure 4.4: Superelevation runoff on reverse curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.5: Superelevation runoff on broken-back curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.6: Typical turning path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-19 Figure 4.7: Truck speeds on grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23 Figure 4.8: Sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25 Figure 4.9: Sight distance on a sag curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-28 Figure 4.10: Cross-section elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-31 Figure 4.11: Verge area indicating location of boulevard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 Figure 4.12: Typical layout of a bus stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-47 Figure 4.13: Bicycle envelope and clearances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-50 Figure 4.14: Collision rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.15: Prediction of utility pole crashes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.16: Typical drain profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-58

LIST OF TABLES Table 4.1: Minimum radii for various values of emax (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 Table 4.2: Design domain for emax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 Table 4.3: Values of superelevation for above min radii of curvature (%): emax = 4 % . . . . . . . . . . . . . . . 4-11 4-11 Table 4.4: Values of superelevation for above min radii of curvature (%): emax = 6 % . . . . . . . . . . . . . . . 4-12 Table 4.5: Values of superelevation for above min radii of curvature (%): emax = 8 % . . . . . . . . . . . . . . . 4-13 Table 4.6: Values of superelevation for above minradii of curvature (%): emax = 10 %. . . . . . . . . . . . . . . 4-13 Table 4.7: Maximum relative gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14 Table 4.8: Lane adjustment factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Table 4.9: Maximum radii for use in spiral transition curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18 Table 4.10: Lengths of grade for 15 km/h speed reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.11: Maximum gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.12: Minimum values of k for crest curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.13: Minimum k-values for barrier sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.14: Minimum k-values for sag curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 Table 4.15: Warrant for climbing lanes. lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-35 Table 4.16: Shoulder widths for undivided rural roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42 Table 4.17: Warrants for pedestrian footways in rural areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 Table 4.18: Cycle lane widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 Table 4.19: Typical widths of roadside elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 Table 4.20: Scour velocities for various materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-57

LIST OF FIGURES

Figure 4.1: 4.1: Dynamics Dynamics of a vehicle vehicle on a curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 Figure 4.2: Methods of distributing of e and f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10 Figure 4.3: Attainment of superelevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Figure 4.4: Superelevation runoff on reverse curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.5: Superelevation runoff on broken-back curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.6: Typical turning path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-19 Figure 4.7: Truck speeds on grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23 Figure 4.8: Sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25 Figure 4.9: Sight distance on a sag curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-28 Figure 4.10: Cross-section elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-31 Figure 4.11: Verge area indicating location of boulevard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 Figure 4.12: Typical layout of a bus stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-47 Figure 4.13: Bicycle envelope and clearances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-50 Figure 4.14: Collision rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.15: Prediction of utility pole crashes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.16: Typical drain profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-58

Chapter 4 ROAD DESIGN ELEMENTS 4.1

INTRODUCTION

4.2

HORIZONTAL ALIGNMENT

Regardless of the philosophy brought to bear on

The horizontal alignment comprises three ele-

the design of a road or the classification of the

ments: tangents, circular curves and the transi-

road in the network, the final design comprises

tions between tangents and curves.

a grouping and sizing of different elements. A vertical curve is an element. The development

Tangents (sometimes referred to as "straights")

of super elevation is an element.

The cross-

have the properties of bearing (direction or

section is heavily disaggregated, comprising a

heading) and and length. length. Circular curves also have

large group of elements. Geometric design thus

two properties; radius and deflection (or devia-

comprises:

tion) angle, these two properties directly leading

•

The selection of elements to be incorpo-

to a third property of interest, namely curve

rated in the design;

length. The basic properties properties of transition transition curves

The sizing of the selected elements, and

are shape and length. Design of the horizontal

• •

Linking the elements into a three-dimen-

alignment includes selection of the values asso-

sional sequence.

ciated with each of these six properties.

The selected, sized and linked elements in

4.2.1 4.2.1

combination represent the final design of a road

Genera Generall Cont Control rols s for horizo horizont ntal al alignment

which, when built, must constitute a network link which will satisfactorily match the criteria of

The horizontal alignment should be as direction-

safe, convenient and affordable transportation

al as possible and consistent with the topogra-

with minimum side effects, simultaneously

phy. However, it is equally important in terms of

addressing needs other than those pertaining

context sensitive design to preserve developed

directly to the movement of people or freight.

properties and areas of value to the community.

A road network network comprises comprises links and nodes. nodes. The

Winding alignments composed of short curves

intersections and interchanges are the nodes

and tangents should be avoided if at all possible

and the roads connecting them the links.

because they tend to cause erratic operation

Intersections and the elements that constitute

and a high consequent crash rate.

them are discussed in Chapter 6, with interchanges being dealt with in Chapter 7.

Most design manuals recommend that minimum radii should be avoided wherever possible, sug-

In this chapter, the various elements involved in

gesting that flat curves (i.e. high values of

link design are discussed.

radius) should be used, retaining the minimum

4-1

e d i u G n g i s e D c i r t e m o e G

for the most critical conditions. The concept of

1 000 metres. If a curve radius is such that the

consistency of design, on the other hand, sug-

curve either has a normal camber or a crossfall

gests that the difference between design speed

of 2 per cent, the limitation on curve length falls

and operating speed should ideally be held to a

away and the curve can be dealt with as though

maximum of 10 km/h, with a 20 km/h difference

it were a tangent.

still representing tolerable design. This could be construed as a recommendation that minimum

Broken-back (also referred to as "flat back")

curvature represents the ideal, which is wholly

curves are a combination of two curves in the

at variance with the historic approach to selec-

same direction with an intervening short tan-

tion of curve radius. What is actually intended

gent.

however is that the designer should seek to

grounds of aesthetics and safety. Not only are

employ the highest possible value of design

broken-back curves unsightly but drivers do not

speed for any given circumstance.

always recognize the short intervening tangent

These should be avoided on the twin

and select a path corresponding to the radius of For small deflection angles, curves should be

the first curve approached, hence leaving the

sufficiently long to avoid the appearance of a

road on the inside and part way along the tan-

kink.

A widely adopted guideline is that, on

gent. On roads in areas with restrictive topog-

minor roads, curves should have a minimum

raphy, such as mountain passes, the designer

length of 150 metres for a deflection angle of 5 O

may have no option but to accept the use of bro-

and that this length should be increased by 30

ken-back curves but should, nevertheless, be

metres for every 1O decrease in deflection

aware of their undesirability. Where the inter-

angle. On major roads and freeways, the mini-

vening tangent is longer than 500 metres, the

mum curve length in metres should be three

appellation "broken-back" is no longer appropri-

times the design speed in km/h. The increase in

ate.

length for decreasing deflection angle also


applies to these roads. In the case of a circular

Reverse curves are also a combination of two

curve without transitions, the length in question

curves but in opposite directions with an inter-

is the total length of the arc and, where transi-

vening short tangent. These curves are aes-

tions are applied, the length is that of the circu-

thetically pleasant but it is important to note that

lar curve plus half the total length of the transi-

the intervening tangent must be sufficiently long

tions.

to accommodate the reversal of superelevation between the two curves.

South African practice recommends an upper limit to the length of horizontal curves. Curves

Although compound curves afford flexibility in

to the left generally restrict passing opportuni-

fitting the road to the terrain and other ground

ties and, furthermore, dependant on their radius,

controls, their use should be avoided outside

operation on long curves tends to be erratic.

intersection or interchange areas. Once they

For this reason, it is desirable to restrict the

are on a horizontal curve, drivers expect the

length of superelevated curves to a maximum of

radius to remain unaltered hence supporting a 4-2

constant speed across the length of the curve.

Obviously this would be of fairly limited duration

A compound curve is thus a violation of driver

in terms of the season and the likelihood of a

expectancy and can be expected to have a cor-

prolonged gradient of this magnitude, whereas

respondingly high crash rate.

the east-west bearing would be a problem all year round and over an extended period of time

4.2.2

Tangents

during the day. The designer should be aware

Two fundamentally different approaches can be

of this problem and, if possible, avoid selecting

adopted in the process of route determination.

bearings that reduce visibility.

In a curvilinear approach, the curves are located

problem cannot be avoided, warning signage

first and thereafter connected up by tangents.

may be considered.

If the dazzle

This approach can be adopted with advantage in mountainous or rugged terrain. A typical con-

In the second instance, a bearing at right angles

sequence of this approach is that curves tend to

to the prevailing wind direction can cause prob-

be long and tangents short.

Because of the

lems for empty trucks with closed load compart-

local topography, the approach to route determi-

ments, e.g. pantechnicons, or passenger cars

nation most frequently adopted in South Africa

towing caravans. On freight routes or routes

is for the tangents to be located first and the

with a high incidence of holiday traffic, the

curves fitted to these tangents thereafter.

designer should seek to avoid this bearing. If not possible, locating the road on the lee of a hill

Bearing

that could then offer shelter from the wind may be an option to be explored in order to offer

Economic considerations dictate that, where

some relief.

other constraints on route location are absent, roads should be as directional as possible. In

Length

consequence, tangents may be located on bearings that have an adverse impact on driver com-

The minimum allowable length of tangent is that

fort and safety. Two conditions require consid-

which accommodates the rollover of superele-

eration.

vation in a reverse curve situation. In the first instance, a combination of gradient, direction of travel and time of day may cause the

It has been found that extremely long tangents,

driver to be dazzled by the sun. The most obvi-

e.g. lengths of twenty kilometres or more, have

ous example is the east-west bearing where the

accident rates similar to those on minimum

vehicle would be moving towards the rising or

length tangents, the lowest accident rate occur-

setting sun. No direction of travel is completely

ring in a range of eight to twelve kilometres.

exempt from this problem. For example, when

This range is recommended for consideration in

travelling at midday in mid-winter in a northerly

fixing the maximum length of tangent on any

direction up a gradient steeper than about eight

route. This maximum is based on the assump-

per cent, the sun can present a problem.

tion of a design speed of 120 km/h or more. At

4-3


lower design speeds, it is necessary to consider

constitute poor design so that, at a greater level

maximum lengths considerably shorter than

of precision than the proposed rule of thumb,

eight to twelve kilometres as discussed below.

three possibilities arise. These are:

•

Case 1 - The length of tangent is less

As a rough rule of thumb (which is adequate for

than or equal to the distance, T min,

planning purposes) the maximum length of tan-

required to accelerate from the operat ing peed appropriate to one curve to that

gent in metres should not exceed ten to twenty times the design speed in kilometres/hour. If the achievable maximum length of tangent across

appropriate to the next curve;

•

Case 2 - The length of tangent allows acceleration to a speed higher than that

the length of the route is regularly greater than

appropriate to the next curve but not as

this guideline value, thought should be given to

high as the desired speed remote from

consideration of a higher design speed. Rules

inhibiting curves, and

of thumb have their limitations and, in this case,

•

Case 3 - The length of tangent, T max, allows acceleration up to desired speed.

application should be limited to design speeds

These cases are intended to be applied

of 100 km/h or less. At a design speed of 120

in areas where curves are to design

km/h or higher, a maximum tangent length of

speeds of 100 km/h or less.

1 200 to 2 400 metres would clearly be meaningless.

To apply the guideline values of allowable speed

As an example of the application of the rule of

variations and differences, it is necessary to

thumb, a design speed of 80 km/h would sug-

estimate the operating speeds on the curves

gest that the maximum length of tangent should

preceding and following a tangent.

be in the range of 800 to 1 600 metres. In this

absence of information specific to South African

range, drivers would tend to maintain a fairly

conditions, the international value of V 85 given

constant speed of about 80 km/h. At greater

in Equation 4.1 will have to be employed.

In the

lengths of tangent, drivers would accelerate to

V85 = 105,31 + 1,62 x 10 -5 x B2 - 0,064 x B

some higher speed only to decelerate at the fol-

(4.1)

lowing curve. This oscillation in speed is inher-

where V85

85th percentile speed

=

(km/h)

ently dangerous as discussed in more detail

e below. d i u Consistency of design dictates that: G • The difference between design speed n and 85th percentile speed, and g i • Variations in 85th percentile speeds s between successive elements e D should be limited as far as possible. Research c has indicated that, ideally, these differences and i r variations should be less than 10 km/h but that t e an acceptable design still results if they are less m than 20 km/h. Differences in excess of 20 km/h o 4-4 e G

B

and

=

Bendiness

=

57 300/R (degrees/km)

θ

=

Deviation angle

L

=

Total length of curve (metres).

The general form for bendiness, B, in the case where the circular curve is bounded by transition curves is B = (Lcl1/2R + Lcr /R +Lcl2/2R) x 180/π x 1000 L

(4.2)

where L

=

LCl1 + LCr + LCl2

where

C

=

with LCl1 and LCl2 being the lengths of the pre-

average passenger car speed (km/h)

ceding and succeeding transition curves and

Q

=

flow (veh/h)

LCr the length of the circular curve.

G

=

gradient (per cent)

D

=

directional split as a

The length of tangents is to be compared with

decimal fraction

the values of Tmin and Tmax that, as suggest-

PT

=

number of trucks in the

ed, are a function of the speeds achievable on

traffic stream as a dec

the curves preceding and following these tan-

imal fraction

gents.

The values are calculated using an

PS

=

number of semi trailers

acceleration or deceleration rate of 0,85 m/s 2

in the traffic stream as

(determined by car-following techniques and

a decimal fraction.

which also corresponds to deceleration without braking) TMIN =

In the absence of significant volumes of traffic V8512 - V8522

and on a level grade, the average speed would,

(4.3)

22,03

according to this equation, be of the order of 120 km/h, suggesting that the 85th percentile speed

and

estimated by Eq. 4.1 is very conservative. TMax = 2(V85Tmax)2-(V851)2-(V852)2 22,03

If the tangent length is shorter than T Min the tan-

(4.4)

gent is non-independent and it is only necessary where V85Tmax =

V851

V852

=

=


for the operating speeds of the two adjacent

on long tangent, i.e.

curves to be within the difference ranges

V85, (km/h)

described above to constitute good or tolerable


design. In essence, acceleration to the operat-

on preceding curve

ing speed of the following curve could take

(km/h)

place on this curve itself. Where it is necessary


to decelerate to the operating speed of the fol-

on following curve

lowing curve, it will be necessary for the driver to

(km/h)

brake in order to achieve the appropriate speed

A tangent has a bendiness of zero so that V85

at the start of the curve. It follows that, on a two-

for TMax is, according to Eq. 4.1, 105,31 km/h.

lane two-way road, tangent lengths shorter than

South African research has derived an expres-

TMin are potentially dangerous.

sion for average speed as given in Eq. 4.5. Where the tangent length is just equal to T Max C

=

143,96 - 10,39 ln Q - 0,04 (G2 -

the vehicle will be able to accelerate from the

5,20) - 18,08 D -33,89 PT -

operating speed of the preceding curve to the

54,15 PS

desired speed and then immediately decelerate

(4.5)

to the operating speed of the following curve. In 4-5


this case, the difference between operating

ceeding curves. These define, in effect, the rel-

speeds on each curve and the desired speed

ative design domain of horizontal curvature on

has to be within the allowable range.

any given road, i.e. the possible range of values of radius of any curve given, the radius of the

In the case of a tangent length falling in the

preceding curve.

range TMin < T < TMax , it will be necessary to calculate the highest operating speed that can

The safety of any curve is dictated not only by

be reached by accelerating at a rate of 0,85

the external factors described above but also by

m/s2 from the operating speed of the first curve,

factors internal to it, namely radius, supereleva-

allowing for a deceleration at the same rate to

tion, transitions and curve widening. Of these

the operating speed of the second. It is the dif-

factors, the most significant is radius as

ference between this maximum operating speed

research carried out in Washington State shows

and the speeds on the adjacent curves that is

consistently that crash frequency increases as

critical.

the curve radius decreases. At present, the best

The maximum operating speed on a

tangent of this length is calculated as

model shows that A = (0,96 L + 0,0245/R - 0,012S) 0,978(3.3 x W - 30)

V85

[11,016(T - TMin) + V8512]0,5

=

for V851 > V852

(4.7)

(4.6)

where A

=

crashes/million vehicles entering from both

4.2.3

Curves

directions

Over the years, various theoreticians have pro-

L

=

curve length (km)

posed a variety of polynomials as the most

R

=

curve radius (km)

desirable forms of horizontal curvature, with

S

=

1,

desirability presumably being determined by the

curves have

aesthetics of the end resultant and usually fr om

been provided

a vantage point not normally available to the driver. Accident history suggests, however, that

W

drivers have enough difficulty in negotiating sim-


if transition

=

0,

otherwise

=

roadway width (lanes plus shoulders) (m).

ple circular curves that have the property of providing a constant rate of change of bearing. It is

Using this relationship, the designer would be

recommended that anything more complex than

able to estimate the merits of increasing the

circular arcs should be avoided, the most note-

radius of a curve. This would presumably be of

worthy exception being the loop ramp on inter-

great benefit in the case where an existing road

changes.

is to be upgraded or rehabilitated.

In the preceding section, relationships between

It is necessary to determine the absolute bound-

operating speed and degree of curvature were

aries of the design domain. The upper bound is

offered,

differences

obviously the tangent in the sense that it has a

between the operating speeds observed on suc-

radius of infinite length. The lower bound is the

as

were

acceptable

4-6

minimum radius for the selected design speed

The side friction factor is a function of the condi-

and this is a function of the centripetal force nec-

tion of the vehicle tyres and the road surface

essary to sustain travel along a circular path.

and varies also with speed. For the purposes of

This force is developed in part by friction

design, it is desirable to select a value lower

between the vehicle's tyres and the road surface

than the limit at which skidding is likely to occur

and in part by the superelevation provided on

and the international general practice is to

the curve. The Newtonian dynamics of the situ-

select values related to the onset of feelings of

ation is illustrated in Figure 4.1.

discomfort. Canadian practice suggests that the

Figure 4.1: Dynamics of a vehicle on a curve The relationship between speed, radius, lateral

side friction factor be taken as

friction and superelevation is expressed by the

f

=

0,21 - 0,001xV

(4.9)

=

vehicle speed (km/h).

relationship: e+f where e

f

=

V2/127 R

=

superelevation( taken

=

(4.8)

where V

as positive when the

For any given speed, it is thus only necessary to

slope is downward

select the maximum rate of superelevation,

towards the centre of

emax, in order to determine the minimum allow-

the curve)

able radius of horizontal curvature for that

lateral or side friction

speed. This selection is based on considera-

factor

tions of the design domain as discussed in the

V

=

speed of vehicle (km/h)

following section.

In practice, four values of

R

=

radius of curvature (m).

emax are used, being 4, 6, 8, and 10 per cent. The minimum radius of curvature appropriate to

This equation is used to determine the minimum

design speeds in the range of 40 km/h to 130

radius of curvature that can be traversed at any

km/h for each of these values of e max is given in

given speed.

Table 4.1.

4-7


Guidelines are offered in the following section

as 8 per cent, provided that this value of super-

for the selection of emax

elevation is used only between intersections and that the superelevation is sufficiently remote

4.2.4

Superelevation

from the intersections for full run-off to be achieved prior to reaching the intersection area.

The selection of the appropriate value of e max is at the discretion of the designer in terms of the

In rural areas, the range of observed speeds is

design domain concept. The higher values of

relatively limited and adequate distance to allow

emax are typically applied to rural areas and the

for superelevation development and runoff is

lower values to the urban environment.

usually available. Climatic conditions may, however, impose limitations on the maximum value


The spatial constraints in urban areas will very

of superelevation that can be applied. Icing of

often preclude the development of high values

the road surface is not a typical manifestation of

of superelevation. Because of congestion and

the South African climate but has been known to

the application of traffic control devices, the

occur in various high-lying parts of the country.

speeds achieved at any point along the road

Heavy rainfall reduces the available side friction

can fluctuate between zero and the posted

and relatively light rain after a long dry spell also

speed - or even higher depending on the local

reduces side friction.

level of law enforcement. Negotiating a curve

to areas where the road surface is polluted by

with a superelevation of 10 per cent at a crawl

rubber and oil spills, as is the case in urban

speed can present a major problem to the driv-

areas and the immediately surrounding rural

er. As a general rule, urban superelevations

areas. Where any of these circumstances are

should not exceed 6 per cent although, in the

likely to occur, a lower value of e

case of an arterial, this could be taken as high

mended. 4-8

This applies particularly

max

is recom-

A lower value of emax should also be considered

•

in a road where steep gradients occur with any

applied to sustain lateral acceleration down to

frequency.

A superelevation of 10 per cent

radii requiring f max followed by increasing e with

would present trucks with some difficulties when

reducing radius until e reaches e max. In short,

they are climbing a steep grade at low speeds.

first f and then e are increased in inverse pro-

As shown in Table 7.3, the combination of a

portion to the radius of curvature;

superelevation of 10 per cent and a gradient of

•

8 per cent has a resultant of 12,8 per cent at

of Method 2 with first e and then f increased in

approximately 45O to the centreline.

inverse proportion to the radius of curvature;

•

Method 2:

Method 3:

Method 4:

Side f riction i s f irst

Effectively t he r everse

As

f or

Method

3,

except that design speed is replaced by aver-

Whatever the value selected for e max, this value

age running speed, and

should be consistently applied on a regional

•

basis. Its selection governs the rate of superel-

Method 5:

Superelevation

and

side friction are in curvilinear relations with the

evation applied to all radii above the minimum.

inverse of the radius of curvature, with values

Variations in emax result in curves of equal

between those of Methods 1 and 3.

radius having different rates of superelevation. Drivers select their approach speeds to curves

These methods of distribution are illustrated in

on the basis of the radius that they see and not

Figure 4.2.

on the degree of superelevation provided.

A

lack of consistency with regard to supereleva-

In terms of the design domain concept, Method

tion would almost certainly lead to differences in

2 has merit in the urban environment. As point-

side friction demand with possibly critical conse-

ed out earlier, provision of adequate superele-

quences.

vation in an environment abounding in con-

Recommended rates of emax are

offered in Table 4.2.

straints such as closely spaced intersections

Distribution of e and f

and driveways is problematic. It is thus sensible

There are a number of methods of distributing e

to make as much use as possible of side fr iction

and f over a range of curves flatter than the min-

before having to resort to the application of

imum for a given design speed. Five methods

superelevation.

are well documented by AASHTO. These are:

drivers operating at relatively low speeds in an

•

superelevation

urban environment are prepared to accept high-

and side friction are directly proportional to the

er values of side friction than they would at high

inverse of the radius;

speeds on a rural road.

Method 1:

Both

4-9

It also should be noted that


Method 5 is recommended for adoption in the

bution for superelevation over the range of cur-

case of rural and high-speed urban roads. In

vature.

practice it represents a compromise between Methods 1 and 4. The tendency for flat to inter-

Tables 4.3, 4.4, 4,5 and 4,6 provide values of e

mediate curves to be overdriven is accommo-

for a range of horizontal radii and values of e max

dated by the provision of some superelevation.

of 4, 6, 8, and 10 per cent respectively.

The superelevation provided sustains nearly all

Superelevation runoff

lateral acceleration at running speeds (assumed to be about 80 per cent of design speed) with


considerable side friction available for greater

In the case of a two-lane road, superelevation

speeds. On the other hand, Method 1, which

runoff (or runout) refers to the process of rotat-

Figure 4.2: Methods of distributing of e and f avoids the use of maximum superelevation for a

ing the outside lane from zero crossfall to

substantial part of the range of curve radii, is

reverse camber (RC) thereafter rotating both

also desirable. Method 5 has an unsymmetrical

lanes to full superelevation. Tangent (or crown)

parabolic form and represents a practical distri-

runoff refers to rotation of the outside lane from 4-10

zero crossfall to normal camber (NC). Rotation

drainage resulting in the possibility of storm

is typically around the centreline of the road

water ponding on the road surface. A further

although

driveway

cause of ponding could be where the centreline

entrances or drainage in the urban environment,

gradient is positive and equal to the relative gra-

may require rotation to be around the inside or

dient. In this case, the inner edge of the road

outside edge. These latter alternatives result in

would have zero gradient over the entire length

the distortion of the vertical alignment of the

of the superelevation runoff.

constraints,

such

as

road centreline and a severe slope on the road edge being rotated, with a potentially unaesthet-

Ponding is extremely dangerous for two rea-

ic end result. In the case of dual carriageway

sons. The more obvious danger is that it can

cross-sections, rotation is typically around the

cause a vehicle to hydroplane, causing a total

outer edges of the median island.

loss of traction and steering ability. If the front wheels are pointing in any direction other than

The designer should be sensitive to the fact that

straight ahead when the vehicle moves out of

zero crossfall implies a lack of transverse

the ponded water, the sudden availability of fric-

Note:

NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the trav-

elled way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-11


tion can lead to a sharp swerve and subsequent

which suggests that there is a maximum accept-

loss of control. The other possibility is that of

able difference between the gradients of the

one front wheel striking the water before the

axis of rotation and the pavement edge.

other, in which case the unbalanced drag could

Experience indicates that relative gradients of

also lead to the vehicle swerving out of control.

0,8 and 0,35 per cent provide acceptable runoff

The designer should therefore endeavour to

lengths for design speeds of 20 km/h and 130

avoid the combination of zero longitudinal gradi-

km/h respectively. Interpolation between these

ent and zero crossfall. A pond depth of 15 mm

values provides the relative gradients shown in

is sufficient to cause hydroplaning. In the case

Table 4.7.

of worn tyres, a lesser depth will suffice. Many States of the United States have opted for


The length of the superelevation runoff section

a standard relative gradient of 1:200, whereas

is selected purely on the basis of appearance,

Canada has elected to use a relative gradient of

Note:

NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the travelled

way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-12

Note:

NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the trav-

elled way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-13


1:400 in the calculation of the length of the

If the relative gradient approach to determina-

superelevation runoff.

tion of runoff length is adopted, this length is

Other widely used

options include adopting the distance travelled

calculated as

in four seconds and previous editions of the

4.10

AASHTO policy suggested the distance travelled in 2 seconds. As can be seen, there is a

where L

=

large degree of arbitrariness attaching to deter-

tion runoff (m)

mination of the length of superelevation runoff.

w

=

The designer can thus vary the relative gradient


length of supereleva

width of one traffic lane, (m)

to accommodate other elements of the design,

n

=

number of lanes rotated

such as the distance between successive

ed

=

superelevation rate

curves or the distance to the following intersec-

(per cent) ∆

tion. It is, however, suggested that the relative

=

gradients offered in Table 4.7 should provide a

relative gradient, (per cent)

pleasing appearance and the designer should at

b

least attempt to achieve relative gradients of a

=

adjustment factor for number of lanes rotated

similar magnitude.

Adjustment factor for number of lanes The gradient of the tangent runout is simply a continuation of whatever relative gradient was

If the above relationship is applied to cross-sec-

adopted for the superelevation runoff.

tions wider than two lanes, the length of the

4-14

superelevation runoff could double or treble and

portion of the runoff located on the tangent and

there may simply not be enough space to allow

the balance on the curve.

for these lengths. On a purely empirical basis,

shown that having about 2/3 of the runoff on the

it is recommended that the calculated lengths

tangent produces the best result in terms of lim-

be adjusted downwards by the lane adjustment

iting lateral acceleration.

factors offered in Table 4.8.

demand, deviation by about 10 per cent from

Experience has

If circumstances

this ratio is tolerable.

Location of superelevation runoff The superelevation runoff and tangent runout

The two extremes of runoff location are:

•

are illustrated in Figure 4.3. Figures 4.4 and 4.5

Full superelevation attained at the

show possible treatments for superelevation

beginning of the curve (BC), and

•

runoff on reverse and broken back curves. In

Only tangent runout attained at the BC.

these cases, the superelevation runoff termi-

Both alternatives result in high values of lateral

nates at a crossfall of two per cent rather than

acceleration and are thus considered undesir-

the more customary zero camber on the out-

able. The preferred option would be to have a

side lane.

Figure 4.3: Attainment of superelevation 4-15


Figure 4.4: Superelevation runoff on reverse curves


Figure 4.5: Superelevation runoff on broken-back curves

4-16

If the circular curve is preceded by a transition

cubic parabola achieves a maximum value and

curve, all of the superelevation runoff should be

then flattens out again and is thus not a true spi-

located on the transition curve.

ral and the lemniscate requires an unacceptable length of arc to achieve the desired radius. The

4.2.5

Transition curves

clothoid, which has the relationship whereby the radius, R, at any point on the spiral varies with

Any vehicle entering a circular curve does so by

the reciprocal of the distance, L, from the start of

following a spiral path. For most curves, this

the spiral, is thus the preferred option.

transition can be accommodated within the lim-

Expressed mathematically, this relationship is

its of normal lane width. At minimum radii for the

R

=

A2/L

(4.11)

design speed, longer transition paths are fol-

where A is a constant called the spiral parame-

lowed and, if these occur on narrow lanes, the

ter and has units of length.

shift in lateral position may even lead to encroachment on adjacent lanes. Under these

The length of a transition curve may be based

circumstances, it may be convenient to shape

on one of three criteria. These are:

the horizontal alignment such that it more accu-

•

Rate of change of centripetal accelera-

rately reflects the path actually followed by a

tion, essentially a comfort factor, varying

vehicle entering the circular curve.

between 0,4 m/s 3 and 1,3 m/s3;

• •

Various curves can be used to provide a transi-

Relative slope as proposed in Table 4.3; or Aesthetics.

tion from the tangent to the circular curve. Whatever form is used, it should satisfy the con-

Since relative slope is applied to curves where

ditions that:

transitions are not provided, its use also for tran-

• •

It is tangential to the straight;

sitioned curves would be a sensible point of

Its curvature should be zero (i.e. infinite

departure.

radius) on the straight;

•

radius corresponding to a specified centripetal

The curvature should increase (i.e.

acceleration can be calculated. These radii are

radius decrease) along the transition;

•

listed in Table 4.9 for an acceleration of 1,3

Its length should be such that, at its

m/s2. There is little point in applying transition

junction with the circular curve, the full

•

For the various design speeds, a

super elevation has been attained;

curves to larger radii where the centripetal

It should join the circular arc tangential-

acceleration would be lower.

ly, and

•

The radius at the end of the transition

Setting out of transitions

should be the same as that of the circular curve.

Application of the spiral has the effect that the Candidate curves are the lemniscate, the cubic

circular curve has to be offset towards its centre.

parabola (also known as Froude's spiral), and

It is thus located between new tangents that are

the clothoid (also known as Euler's spiral). The

parallel to the original tangents but shifted from

4-17


4.2.6

them by an amount, s, known as the shift. The

Lane widening

value of the shift is given by When vehicles negotiate a horizontal curve, the s

=

L2/2R

rear wheels track inside the front wheels. In the

(4.12)

case of semi trailers with multiple axles and where L

R

=

=

selected length of tran

pivot points, this off-tracking is particularly

sition curve (m)

marked. The track width of a turning vehicle,

radius of circular curve (m)

also known as the swept path width, is the sum of the track width on tangent and the extent of

The starting point of the spiral is located at a dis-

off-tracking, with the off-tracking being a func-

tance, T, from the Point of Intersection (PI) of the

tion of the radius of the turn, the number and

original tangents with

location of pivot points and the length of the wheelbase between axles. The track width is


T

=

(R+s) tan θ/2 + L/2

calculated as

(4.13) U where θ

=

=

u + R - (R2 - ΣLi2)0,5 (4.15)

deviation angle of the circular curve

where U

=

track width on curve (m)

u

=

track width on tangent (m)

The most convenient way to set out the spiral is

R

=

radius of turn (m)

by means of deflection angles and chords and

Li

=

wheel base of design

the deflection angle for any chord length, l, is

vehicle between suc-

given as

cessive axles and pivot

a

=

l2/6RL x 57,246 (4.14)

points (m).

4-18

Strictly speaking, the radius, R, should be the

FA

=

[R2 + A(2L + A)]0,5 - R

radius of the path of the midpoint of the front axle.

(4.16)

For ease of calculation, however, the

radius assumed is that of the road centreline.

where FA

=

width of front overhang (m)

The front overhang is the distance from the front

R

=

radius of curve (m)

axle of the vehicle to the furthest projection of

A

=

front overhang (m)

the vehicle body in front of the front axle. In the

L

=

wheel base of single

case of the turning vehicle, the width of the fr ont

unit or tractor (m)

overhang is defined as the radial distance between the path followed by the outer front

The width of the rear overhang is the radial dis-

edge of the vehicle and the tyre path of the outer

tance between the outside edge of the inner

front wheel. The width of the front overhang is

rearmost tyre and the inside edge of the vehicle

calculated as

body. In the case of a passenger car this dis-

Figure 4.6: Typical turning path 4-19


tance is typically less than 0,15 m. The width of

WC

=

N (U + C) + FA(N - 1) + Z

truck bodies is usually the same as the wheel-

(4.18)

base width so that the width of the rear overhang is zero.

where N

=

number of lanes

and the other variables are as previously A typical turning path is illustrated illustrated in Figure 4.6. 4.6.

defined.

Turning paths for numerous vehicles are provided in the 2000 edition of the AASHTO Policy on

As a general rule, values of curve widening,

geometric design of highways and streets and

being (WC - W) where W is the width of the trav-

the designer is directed towards Exhibits 2-3 to

elled way on tangent sections, that are less than

2-23 of that document.

0,6 m are disregarded. Lane widening is thus generally not applied to curves with a radius

It is necessary to provide an allowance, C, for

greater than 300 metres, regardless of the

lateral clearance between the edge of the road-

design speed or the lane width.

way and the nearest wheel path, and for the body clearance between passing vehicles.

Widening should transition gradually on the

Typical values of C are:

approaches to the curve so that the full addi-

•

0,60 m for a travelle travelled d way width width of 6,0

tional width is available at the start of the curve.

m;

Although a long transition is desirable to ensure

0,75m 0,75m for a travel travelled led way wid width th of 6,6 6,6

that the whole of the travelled way is fully

m, and

usable, this results in narrow pavement slivers

0,90 0,90 m for a travel travelled led way wid width th of of 7,4 m.

that are difficult, and correspondingly expen-

• •

sive, to construct. In practice, curve widening widening is


A further allowance, allowance, Z, is provided to accommoaccommo-

thus applied over no more than the length of the

date the difficulty of manoeuvring on a curve

super elevation runoff preceding the curve. For

and the variation variation in driver driver operation. This addi-

ease of construction, the widening is normally

tional width is an empirical value that varies with

applied only on one side of the road. This is

the speed of traffic and the radius of the curve.

usually on the inside of the curve to match the

It is expressed as

tendency for drivers to cut the inside edge of the travelled way.

Z

=

0,1(V/R0,5)

(4.17)

In terms of usefulness and aesthetics, a tangent where V

=

design speed of the

transition edge should be avoided. A smooth

road (km/h)

graceful curve is the preferred option and can be adequately achieved by staking a curved

By combining Eqs 4.12, 4.13 and 4.14 with the

transition by eye. eye. Whichever approach approach is used, used,

clearance allowances, allowances, C and Z, the width of the

the transition ends should avoid an angular

travelled way can be calculated as

break at the pavement edge.

4-20

Widening is provided to make driving on a curve

constant rate rate of change of bearing. bearing. It thus has

comparable with that on a tangent. On older

a certain academic appeal.

roads with narrow cross-sections and low design speeds and hence sharp curves, there

The general equation of the parabola is

was a considerable need for widening on

y = ax2 + bx +c,

curves. Because of the inconvenience inconvenienc e attached

from which it follows that the gradient, dy/dx, at

to widening the surfacing of a lane, it follows that

any point along the curve is expressed as 2ax +

the required widening may not always have

b and the rate of change of gradient, d 2y/dx2, is

been provided. Where a road has to be rehabil-

2a. This has the meaning of extent of change

itated and it is not possible to increase the

over a unit distance.

radius of curvature, the designer should consid-

express the rate of change in terms of the dis-

er the need for curve widening.

tance required to effect a unit change of gradi-

Normal usage is to

ent. This expression expression is referred to as the KIn the case of an alignment where curves in

value of the the curve and is equal to 1/2a. It fol-

need of widening of the travelled way follow

lows that the length of a vertical curve can be

each other in quick succession, the inconven-

conveniently conveniently expressed as being

ience associated with the application of curve widening can be avoided by constructing the

L

=

AxK

(4.19)

entire section of road, including the intervening tangents, to the additional width.

4.3

where L

=

curve length (m)

A

=

algebraic difference

VERTICAL ALIGNMENT

between the gradients on either side of the

Vertical alignment comprises grades (often

curve

referred to as tangents) and vertical curves.

K

4.3. 4.3.1 1

Grades Grad es have ha ve the t he proper pr operties ties of length length and and gradie gradient, nt,

=

rate of change

Gene Genera rall contr control ols s for ver verti tica call alignment

invariably expressed as a percentage, representing the height in metres gained or lost over a horizontal distance of 100 metres.

On rural and high-speed urban roads, a smooth grade line with gradual changes, which are con-

Curves may be either circular or parabolic, with

sistent with the class of the road and the char-

South African practice favouring favouring the latter. latter. The

acter of the terrain, is preferable to an alignment

practical difference between the two forms is

with numerous breaks and short lengths of

insignificant in terms of actual roadway levels

grades and curvature. A series of successive,

along the centreline. The parabola has the

relatively sharp crest and sag curves creates a

property of providing a constant rate of change

roller coaster or hidden dip profile which is aes-

of gradient with distance, which is analogous to

thetically theticall y unpleasant.

the horizontal circular curve, which provides a

safety concern, although, at night, the loom of 4-21

Hidden dips can be a


approaching headlights may provide a visual

Where the total change of gradient across a ver-

clue about oncoming vehicles.

Such profiles

tical curve is very small, e.g. less than 0,5 per

occur on relatively straight horizontal alignments

cent, the K-value necessary to achieve the min-

where the road profile closely follows a rolling

imum length of curve would be high.

natural ground line.

these circumstances, the vertical curve could be

Under

omitted altogether without there being an A broken-back grade grade line, which which consists of two

adverse visual impact.

vertical curves in the same direction with a short length of intervening tangent, is aesthetically The vertical alignment design should not be car-

unacceptable, particularly in sags where a full

ried out in isolation but should be properly coor-

view of of the profile is possible. possible. A broken plank

dinated with the horizontal alignment as dis-

grade line, where two long grades are connect-

cussed later. In addition to the controls imposed

ed by a short sag curve, is equally unaccept-

on the grade line by the horizontal alignment,

able. As a general rule, the length of a curve (in

the drainage of the road may also have a major

metres) should not be shorter than the design

impact on the vertical alignment. The top of a

speed in km/h. In the case of freeways, freeways, the min-

crest curve and the bottom of a sag imply a zero

imum length should not be less than twice the

gradient and the possibility of ponding on the

design speed in km/h and, for preference,

road surface. Where water flow off the road road sur-

should be 400 metres or longer to be in scale

face is constrained by kerbs, the gradient

with the horizontal horizontal curvature. curvature. The broken-back broken-back

should be such that longitudinal flow towards

and broken-plank curves are the vertical coun-

drop inlets or breaks in the kerb line is support-

terparts of the horizontal broken-back curve and

ed.

the long tangent/small radius curve discussed earlier.

On lower-speed lower-spee d urban roads, drainage

design may often control the grade design.

The only difference between them is

that these forms of vertical alignment are, at There are, to date, no specific guidelines on

least, not dangerous. dangerous.

consistency of vertical alignment in terms of the


In theory, vertical curves in opposite directions

relative lengths of grades and values of vertical

do not require grades between them. In prac-

curvature, as is the case in horizontal horizontal alignment.

tice, however, the outcome is visually not suc-

However, where grades and curves are of

cessful. The junction junction between between the the two curves

approximately equal lengths, the general effect

creates the impression of a sharp step in the

of the grade line tends to be pleasing.

alignment, downwards where a sag curve fol-

4.3.2

lows a crest and upwards where the crest curve

Grades

follows the sag. A short length of grade between the two curves will create the impression of con-

The convention adopted universally is that a

tinuous, smoothly flowing vertical curvature.

gradient that is rising in the direction of increas-

The length of the intervening grade in metres

ing stake value is positive and a descending

need not be more than the design speed in km/h

gradient negative.

to achieve this effect.

interlinked in that steep gradients have an 4-22

Gradient and length are

adverse effect on truck speeds and hence on

length of grade" typically taken as being the dis-

the operating characteristics of the entire traffic

tance over which a speed reduction of 15 km/h

stream. This effect is not limited to upgrades

occurs. For a given gradient, lengths less than

because truck operators frequently adopt the

the critical length result in acceptable operation

rule that speeds on downgrades should not

in the desired range of speeds.

exceed those attainable in the reverse direction.

desired freedom of operation is to be maintained

Where the

on grades longer than the critical length, it will It is desirable that truck speeds should not

be necessary to consider alleviating measures

decrease too markedly. Apart from the opera-

such as local reductions of gradient or the pro-

tional impact of low truck speeds, it has also

vision of extra lanes.

been established that there is a strong correlation between crash rates and the speed differ-

Local research indicates that the 85th percentile

ential between trucks and passenger cars.

mass/power ratio is of the order of 185 kg/kW.

American research indicates that crash rates for

The performance of the 85th percentile truck is

speed reductions of less than 15 km/h fluctuate

illustrated in Figure 4.7. The critical lengths of

between 1 and 5 crashes per million kilometres

grade for a speed reduction of 15 km/h are

of travel increasing rapidly to of the order of 21

derived from these performance curves and are

Figure 4.7: Truck speeds on grades crashes per million kilometres of travel for a

shown in Table 4.10.

speed reduction of 30 km/h. In the absence of South African research, it is presumed that a

As suggested earlier, grades longer than those

similar trend would manifest itself locally,

given in Table 4.10 may require some form of

although probably at higher crash rates.

alleviating treatment. One such treatment is the

For

these reasons, reference is made to the "critical

provision of climbing lanes. This is discussed in 4-23


Section 4.4.2. Stepping the grade line, i.e. by

readily be achieved under the three sets of cir-

inserting short sections of flatter gradient, as an

cumstances. Other factors that should be borne

alleviating treatment is sometimes offered as

in mind in selecting a maximum gradient

relief to heavy trucks at crawl speeds on steep

include:

gradients. In practice, this has proved to be

•

would suggest a reduction in maximum

ineffective because drivers of heavy trucks sim-

gradient in order to maintain an accept

ply maintain the crawl speed dictated by the

able Level of Service;

steeper gradient in preference to going through

•

the process of working their way up and down

costs, being the whole-life cost of the road and not merely its initial construc-

through the gears.

tion cost;

•

Maximum acceptable gradients shown in Table

property, where relatively flat gradients in a rugged environment may result in

4.11 are dictated primarily by the topography

high fills or deep cuts necessitating the

and the classification of the road. Topography is

acquisition of land additional to the nor-

described as being flat, rolling or mountainous

mal road reserve width;

which is somewhat of a circular definition in the


traffic operations, where high volumes

• •

sense that what is really being described is not

environmental considerations; and adjacent land use in heavily developed or urban areas

the topography itself but the gradients that can

4-24

It is the designer's responsibility to select a max-

should be considered is of the order of 0,5 per

imum gradient appropriate to the project being

cent.

designed. The values offered in Table 4.11 are

kerbing, gradients should not be less than 0,5

thus only intended to provide an indication of

per cent. If the grade is longer than 500 metres,

gradients appropriate to the various circum-

increasing the camber to 2,5 or 3,0 per cent

stances.

should be considered. The latter value of cam-

It is recommended that, even without

ber should only be considered in areas subject Maximum gradients on freeways should be in

to heavy rainfall because it may give rise to

the range of three to four per cent regardless of

problems related to steering and maintaining the

the topography being traversed. Lower order

vehicle's position within its lane.

roads have been constructed to gradients as steep as twenty per cent but it is pointed out that

Crest curves are often in cut and, for the mini-

compaction with a normal 12/14 tonne roller is

mum value of K for a design speed of 120 km/h,

virtually impossible on a gradient steeper than

the gradient would be at a value of less than 0,5

about twelve per cent. It is recommended that

per cent for a distance of 55 metres on either

this be considered the absolute maximum gradi-

side of the crest. Over this distance, channel

ent that can be applied to any road.

grading should be applied to the side drains. On sag curves, the distance over which t he longitu-

The minimum gradient can, in theory, be level,

dinal gradient is less than 0,5 per cent is 26

i.e. zero per cent. This could only be applied to

metres on either side of the lowest point at the

rural roads where storm water would be

minimum K-value.

removed from the road surface by the camber and allowed to spill over the edge of the shoul-

Varying the camber between 2 per cent and 3

der. If used on a road that is kerbed, channel

per cent over a distance of about 80 metres will

grading would have to be employed. Given the

provide an edge grading at 0,5 per cent in the

limits of accuracy to which kerbs and channels

case where the centreline gradient is flat. As an

can be constructed, the flattest gradient that

alternative means of achieving adequate

Figure 4.8: Sight distance on crest curves 4-25


drainage, this is more useful in theory than in

by the required sight distance is contained with-

practice because shaping and compacting the

in the length of the vertical curve.

road surface to have this wind in it is extremely

If the curve length is shorter than the required

difficult. This method is not unknown but is not

sight distance, lesser values of K can be

recommended because of the construction

employed as indicated by Equation 4.21.

problem. (4.21)

4.3.3

Curves where K

=

As the parameter, K, has been described as the

Distance required for a 1 % change of gradient (m)

determinant of the shape of the parabolic curve,

S

=

Stopping sight distance

it follows that some or other value of K can be

for selected design

determined such that it provides adequate sight

speed (m)

distance across the length of the curve. Sight

h1

=

Driver eye height (m)

distance is measured from the driver eye height,

h2

=

Object height (m)

h1, to a specified object height, h 2. In the case

A

=

Algebraic difference in

of a crest curve, the line of sight is taken as

gradient between the

being a grazing ray to the road surface, as illus-

approaching and depart-

trated in Figure 4.7.

ing grades (%) The values of K offered in Table 4.12 are based

Vertical curvature for stopping sight distance

on a curve length longer than the required stop-

The required value of K is derived from the

ping distance with a driver eye height of 1,05

equation of the parabola as indicated in

metres and various heights of object as dis-

Equation 4.20.

cussed in Chapter 3. K=

S2 200(h 10,5 + h20,5)2

where K


S

=

=

(4.20)

Vertical curvature for passing sight distance

Distance required for a

Similar calculations can be carried out based on

1 % change of gradient

passing sight distance. High values of K result

(m)

so that, in the situation where the crest of the

Stopping sight distance

curve is in cut, the increase in volumes of exca-

for selected design

vation will be significant. Although the designer

speed (m)

should seek to provide as much passing sight distance as possible along the length of the

h1

=

Driver eye height (m)

h2

=

Object height (m)

A

=

Algebraic difference in

road, it may be useful to shorten the crest curve in order to increase the lengths of the grades on

gradient between the approaching and depart

either side rather than to attempt achieving

ing grades (%).

passing sight distance over the crest curve itself.

This relationship applies to the condition where4-26

Vertical curvature for barrier sight dis-

specifically in cases where the actual speed dif-

tance

ferentials between the overtaking and the overtaken vehicles are greater than those applied in

On undivided roads, barrier sight distance (also

the derivation of passing sight distances. K val-

referred to as non-striping sight distance) indi-

ues of crest curvature corresponding to barrier

cates whether no-passing pavement markings

sight distance are offered in Table 4.13.

are required. Barrier sight distance is shorter than passing sight distance.

Sag curves

The designer

should attempt to provide barrier sight distance or more wherever possible because passing

During the hours of daylight or on well-lit streets

manoeuvres can often be completed in less

at night, sag curves do not present any prob-

than the calculated passing sight distance

lems with regard to sight distance. Under these

4-27


circumstances, the value of K is determined by

existing pedestrian bridges, a clearance of 5,6

considerations of comfort, specifically the

metres may be accepted.

degree of vertical acceleration involved in the

because pedestrian overpasses are relatively

change in gradient. The maximum comfortable

light structures that are unable to absorb severe

vertical acceleration is often taken as 0,3 m/s 2.

impact and are more likely to collapse in such an event.

This is suggested

The increased vertical clearance

Where the only source of illumination is the

reduces the probability of damage to the struc-

vehicle's headlights, the line of sight is replaced

ture and improves the level of safety for pedes-

by a line commencing at headlight height, taken

trians using it.

as being 0,6 metres, and with a divergence angle of 1O relative to the grade line at the posi-

The criteria of comfort, sight distance and verti-

tion of the vehicle on the curve. This situation is

cal clearance lead to a minimum desirable

illustrated in Figure 4.9.

length of sag curve. A further criterion is that of drainage, whereby a minimum distance with a

Although not a frequent occurrence, sight dis-

gradient of less than 0,5 per cent is desired.

tance on a sag curve may be impaired by a structure passing over the road. Checking the Where the stopping sight distance is less than

available sight distance at an undercrossing is best made graphically on the profile.

the length of the curve, the value of K is given

In this

by Equation 4.22.

case, the line of sight is a grazing ray to the soffit of the structure. The selected clearance is

K

thus of interest. Clearances are typically taken

=

____ S2___

(4.22)

120 + 3,5S

as 5,2 metres measured at the lowest point on the soffit. In the case of pedestrian bridges, the clearance is 5,9 metres while, in the case of


where S

=

Figure 4.9: Sight distance on a sag curve 4-28

stopping distance (m)

4.4

The criterion of comfort, as expressed in

CROSS-SECTIONS

Equation 4.23, provides K-values roughly half of those dictated by considerations of stopping

The prime determinants of cross-section design

sight distance.

are:

• K

=

V2 / 395

where V

=

design speed (km/h)

The function that the road is intended to serve;

(4.23)

•

The nature and volume of traffic to be accommodated; and

•

K-values for sag curves, as determined by

The speed of the traffic.

headlight distance and comfort, for the case where the sight distance is less than the length

Road function refers to a spectrum of needs

of the curve, are given in Table 4.14

ranging

from

accessibility

to

mobility.

Furthermore, a road can be classified on a variIn both crest and sag curves, K-values dictated

ety of different bases. A common feature of

by sight distance, where the curve length is

these considerations is that they largely concern

greater than the sight distance, can be used

addressing the needs of the occupants of a

throughout.

The alternative case, where the

moving vehicle. These needs may find expres-

sight distance is longer than the curve length,

sion in a desire for ready access to or from a

generally offers a lower K-value.

property adjacent to the road, freedom to manoeuvre in a terminal or intersection area or

K-values appropriate to headlight distance

for high-speed long distance travel. High-speed

should be used in rural areas and also where

traffic requires more space than relatively slow-

street lighting is not provided.

moving traffic. Space takes the form of wider

Where street

lighting is provided, the lower K-values associ-

lanes, wider shoulders and (possibly) the inclu-

ated with the comfort criterion may be adopted.

sion of a median in the cross-section.

4-29


All these needs have to be met in terms of over-

these lanes or, as a further development, to pro-

all objectives of safety, economy, convenience

vide cycle paths adjacent to or, for preference,

and minimum side effects.

removed from the travelled lanes.

In urban areas, road functions also have to

Although the horizontal and vertical alignments

include considerations of living space. People

are disaggregated in the sense that they are a

enjoy casual encounters, meeting people on

combination of tangents and curves, the cross-

neutral territory, as it were, without the obligation

section is heavily disaggregated, comprising a

of having to act as host or hostess in the home.

multitude of individual elements.

The sidewalk café, the flea market and window-

ments are illustrated in Figure 4.10. Design is

shopping all have to be accommodated within

thus concerned primarily with the selection of

the road reserve. All of these activities impact

elements that have to be incorporated within the

on the cross-section, which has to be designed

cross-section, followed by sizing of these indi-

accordingly.

vidual elements.

Traffic does not exclusively comprise motorised

In spite of this disaggregated approach to

vehicles. In developing areas, it may be neces-

design, there are numerous combinations of

sary to make provision for animal-drawn vehi-

elements that occur frequently. Cases in point

cles and, in this context, developing areas are

are:

not necessarily exclusively rural. The volume of

• • •

motorised vehicles will have an impact on the design of the cross-section with regard to the

These ele-

Two-lane two-way roads; Two-plus-one roads; Four-lane undivided and divided roads, and

number of lanes that have to be provided. High

•

volumes of moving vehicles will generate a

Four, six (or more) lane freeways.

need for special lanes such as for turning, passEach of these composite cross-sections leads to

ing, climbing or parking.

the development of a standard road reserve


In urban areas, the presence of large numbers

width that often is enshrined in legislation. In

of pedestrians will require adequate provision to

this section, the individual elements rather than

be

the composite cross-section will be discussed.

made

in

terms

of

sidewalk

widths.

Pedestrians are also to be found on rural roads.

4.4.1

On rural roads, speeds are high so that crashes

General controls for cross-sections

involving pedestrians are inevitably fatal. It is Safety is a primary consideration in the design

thus sensible to make at least modest provision

of the cross-section. The safety of the road user

for pedestrians on rural roads, even though their

refers to all those within the road reserve,

numbers may be low.

whether in vehicles or not. Cyclists can often be accommodated on the normal travelled lanes but, when the number of

Wide lanes supposedly promote the safety of

cyclists increases, it may be necessary to widen

the occupants of vehicles although current evi4-30

dence suggests that there is an upper limit

other protected land use. During the planning

beyond which safety is reduced by further

process, it is prudent to attempt to acquire addi-

increases in lane width. The reverse side of the

tional road reserve width to allow for improve-

coin is that wide lanes have a negative impact

ments to traffic operations, auxiliary lanes, wider

on the safety of pedestrians attempting to cross

pedestrian areas, cycle paths as well as for pro-

the road or street. South Africa has a particu-

vision of utilities, streetscaping and mainte-

larly bad record in terms of pedestrian fatalities,

nance considerations.

which account for approximately half of the total

not be taken to the extreme, where large tracts

number of fatalities. In devising safe cross-sec-

of land are unnecessarily sterilised in anticipa-

tions, it is therefore necessary to consider the

tion of some or other future eventuality.

This should, however,

needs of the entire population of road users and not just those in vehicles.

The location of existing major utilities, which may be either above or below ground, and diffi-

In urban areas, it is necessary to make provision

cult or costly to relocate is a fairly common

for boarding and alighting public transport pas-

design control in urban areas. In particular the

sengers, disabled persons and other non-vehic-

location of aboveground utilities in relation to

ular users of the facility in addition to accommo-

clear zone requirements should be carefully

dating pedestrians and cyclists. In these areas,

considered.

design speed usually plays a lesser role in the design of the cross-section.

In the discussion that follows, dimensions are essentially discussed in terms of new or "green

Land availability is of particular importance in

field" designs. Very often, however, the design-

urban areas. Land for the road reserve may be

er does not have this level of fr eedom of choice.

restricted because of the existence of major

For example, rehabilitation projects often

buildings or high cost of acquisition or some or

require the creation of additional lanes without

Figure 4.10: Cross-section elements 4-31


the funding or space available for additional

Similarly, a fourth lane on a 13,4 metre wide

construction. The choice then is one of either

cross-section would imply lane widths of 3,1

accepting a lane or shoulder width that is nar-

metres and a zero width of shoulder if the shoul-

rower than would be desired or foregoing the

der rounding of 0,5 metres is to be maintained.

additional lanes.

The traffic volumes necessitating consideration of a four-lane cross-section would render such a

As discussed below, the preferred cross-section

configuration of lanes and shoulders highly

for a two-lane two-way road has a total width of

undesirable.

13,4 metres. This comprises lane widths of 3,7

4.4.2

metres and usable shoulder widths of 2,5

Basic Lanes

metres plus a 0,5 metre allowance for shoulder Basic lanes are those that are continuous from

rounding. If a cross-section of this width already

one end of the road to the other. The number of

exists and it is deemed necessary to incorporate

lanes to be provided is largely determined by

a climbing lane without incurring extra construc-

traffic flow and the desired Level of Service that

tion costs, this can be achieved by accepting a

the road is to provide. Reference is thus to the

climbing lane width of 3,1 metres with the adja-

Highway Capacity Manual.

cent shoulder having a width of 1,0 metres. The through lanes have to be reduced to a width of

The anticipated traffic speed offers an indication

3,4 metres each and the opposite shoulder to a

of the required width of lane. Lane widths typi-

width of 1,5 metres thus allowing for the shoul-

cally used are 3,1 metres, 3,4 metres and 3,7

der rounding of 0,5 metres on both shoulders.

metres. Research has indicated that the crash rate starts to show a marginal increase above a

Many of the older cross-sections have a total

lane width of 3,6 metres. It is accordingly rec-

paved width between shoulder breakpoints of

ommended that widths significantly greater than

12,4 metres, comprising lanes with a width of

3,7 m should not be employed.

3,4 metres, shoulders with a width of 2,3 metres

authorities use lane widths of 5,4 metres on

and 0,5 metres of shoulder rounding on both

major routes.

sides. Three lanes with a width of 3,1 metres


Some local

The intention is apparently to

make provision for parking or cycle traffic with-

each would allow for shoulders that are approx-

out demarcating the lanes as such. In practice,

imately 1 metre wide and 1 metre of shoulder

passenger cars are very inclined to use these

rounding. It is suggested that the dimensions

lanes as though they were two unmarked lanes

offered for the three-lane cross-section consti-

2,7 metres in width. Although this is a very eco-

tute an irreducible minimum and can be consid-

nomical method for providing adequate capaci-

ered as a palliative measure only over a rela-

ty, the safety record of such cross-sections may

tively short distance, e.g. to accommodate a

be suspect and their use should be discour-

climbing lane. Accommodating a third lane on

aged.

cross-sections narrower than 12,4 metres between shoulder breakpoints should not be

The narrowest width recommended for consid-

essayed.

eration (3,1 metres) allows for a clear space of 4-32

300 mm on either side of a vehicle 2,5 metres

shoulder, or a cross fall, being a slope from one

wide. This width would only be applied to roads

edge of the travelled way to the other edge.

where traffic volumes and/or speeds are expect-

These slopes are provided to ensure drainage

ed to be low.

of the road surface and are typically at two per cent although, in areas where high rainfall inten-

Where traffic volumes are such that a multi-lane

sities are likely to occur, the slope could be

or divided cross-section is required, 3,7 metres

increased to as much as three per cent.

would be a logical width to adopt. High speeds

4.4.3

would also warrant this width because, on a nar-

Auxiliary lanes

rower lane, momentary inattention by a driver could easily cause a vehicle to veer into the path

Auxiliary lanes are located immediately adjacent

of another. Intermediate volumes and speeds

to the basic lanes. They are generally short and

are adequately accommodated by a lane width

are provided only to accommodate some or

of 3,4 metres.

other special circumstance.

In the case of a rehabilitation or reconstruction

Auxiliary lanes are often used at intersections.

project, it may be necessary to add lanes to the

They can be turning lanes, either to the left or to

cross-section. The cost of the additional earth-

the right, or through lanes. The turning lanes

works may however be so prohibitive that the

are principally intended to remove slower vehi-

funds required to upgrade the road to full 3,7

cles, or stopped vehicles waiting for a gap in

metre lanes and 3,0 metre shoulders may sim-

opposing traffic, from the through traffic stream

ply not be available. The options available to

hence increasing the capacity of the through

the road authority are then either not to upgrade

lanes.

the road at all or to accept some lesser widths of

employed at signalised intersections to match

lanes and/or shoulders. It is suggested that a

the interrupted flow through the intersection

stepwise approach to the problem, first reducing

area with the continuous flow on the approach

the available width of shoulder and thereafter

lanes.

considering reductions in lane width, could be

lanes are discussed in depth in Chapter 6.

adopted by.

Auxiliary lanes are also employed at inter-

A 3,4 metre lane is reasonably

Through auxiliary lanes are often

The application and design of these

safe, even at a speed of 120 km/h, but is not a

changes.

comfortable solution when applied over an

intended to achieve lane balance where turning

extended distance. A reduction in the posted

volumes are sufficient to warrant multi-lane on-

speed limit may be desirable if the lesser lane

and off-ramps. They may also be used between

width is applied over several kilometres. At still

closely spaced interchanges to support weav-

lower lane widths, it may be advisable to provide

ing, principally replacing a merge-diverge with a

a posted speed limit of 100 km/h even for rela-

Type A weave. Auxiliary lanes at interchanges

tively short sections of road.

are discussed in Chapter 7.

Travelled lanes have either a camber, being a

Climbing lanes and passing lanes are auxiliary

slope from the centreline towards the outside

lanes employed on network links, i.e. other than 4-33

These are through lanes and are


at intersections and interchanges.

Climbing

last-second manoeuvres occur. In the case of

lanes are often referred to as truck lanes,

merging, such manoeuvres can be extremely

crawler lanes, overtaking lanes or passing

hazardous. Reference should be made to the

lanes. The function of climbing lanes is, howev-

SADC Road and Traffic Signs Manual for the

er, very different from that of passing lanes.

recommended signage and road markings.

Climbing lanes remove slower vehicles from the traffic stream and have the effect of reducing the number of passenger car equivalents (PCE's) in the stream.

If the reduction is sufficient, the

Level of Service (LOS) on the grade will match that on the preceding and succeeding grades. Passing lanes also remove slower traffic from the stream but, in this case, the objective is platoon dispersal, thus supporting an increase in the capacity of the road. In short, the climbing lane seeks to match the LOS on a steep grade to that on the preceding and following flatter grades, whereas the passing lane improves the capacity of the road as a whole. The ultimate demonstration of the latter circumstance is the four-lane road, which could be described as a two-lane road with continuous passing lanes in both directions.

The lane drop should not be located so far from the end of the need for the auxiliary lane that drivers accept the increase in number of lanes and do not expect the reduction. Furthermore, the location should not be such that the lane drop is effectively concealed from the driver. Concealment can arise from location immediately beyond a crest curve. An "architectural" approach which locates the lane drop on a horizontal curve and selects curve radii that result in a smooth transition from two lanes to one across the length of the curve is a particularly subtle form of concealment that should be avoided at all costs. To simplify the driving task, the lane drop should be highly visible to approaching traffic and drivers should not be subjected to successive decision points too quickly implying that lane drops should be

From a safety point of view, it is important that drivers are made aware of the start and, more particularly, the end of an auxiliary lane. The basic driver information requirements in the latter

removed from other decision points. It is possible that the shoulder adjacent to an auxiliary lane could be very narrow. However, for emergency use it is recommended that a

three metre wide surfaced shoulder be provided e case are: Indication of the presence of a lane d i • along and extending downstream of the taper u drop; for a distance equal to the stopping sight dis G • Indication of the location of the lane tance for the design speed of the road. n drop; and g i • Indication of the appropriate action to Climbing lanes s be undertaken. e D Four types of warrants for climbing lanes are in c It has been observed that, without adequate i use. These are: r t signposting and road markings indicating the Reduction of truck speed through a • e given amount or to a specified speed; m presence of a lane drop to drivers, erratic and o 4-34 e G

• • •

Reduction in truck speed in association

before the climbing lane is warranted.

The

with a specified volume of tr affic;

speed reduction applied is 20 km/h from an ini-

Reduction in LOS through one or more

tial 80 km/h. The volume warrant is given in

levels, and

Table 4.15 below.

Economic analysis. Truck speed reductions without reference to

The first three focus on the performance of

traffic volumes have merits in terms of safety.

trucks and infer some or other impact on pas-

Their principal benefit lies in reduction in the

senger vehicles. The fourth directly quantifies

speed differential in the through lane, thereby

the effect of slow moving vehicles (which need

reducing the probability of the occurrence of a

not necessarily be exclusively truck traffic) on

crash. It is, however, theoretically possible that

the traffic stream in terms of delay over the

a climbing lane would be considered warranted

design life of the climbing lane and compares

merely because it would lead to the required

the benefit of the removal of delay with the cost

truck speed reduction even if total traffic vol-

of providing and maintaining the climbing lane.

umes were very low with virtually no trucks in Speed reductions adopted internationally vary in

the traffic stream. The addition of a volume war-

the range of 15 km/h to 25 km/h and are usual-

rant increases the likelihood of a reasonable

ly intended to be applied to a single grade. The

economic return on the provision of a climbing

most widely occurring value of speed reduction

lane.

is 15 km/h, based on considerations of safety.

warrants such as those described above are not

The Australian approach bases the need for

intended to be - or are ever likely to be - fully

climbing lanes on examination of a considerable

economic.

It is conceded that performance-based

length of road. The justification for climbing lanes is based on traffic volume, percentage of

Level of Service is a descriptor of operational

trucks and the availability of passing opportuni-

characteristics in a traffic stream. An important

ties along the road. Speed reduction is to 40

feature is that it is purely a representation of the

km/h and not by 40 km/h.

driver's perception of the traffic environment and

South African practice, as described in TRH17,

is not concerned with the cost of modifying that

uses a combination of the speed and traffic vol-

environment. The warrants suggested by the

ume as a warrant, requiring both to be met

Highway Capacity Manual are: 4-35


•

•

A reduction of two or more Levels of

climbing lanes are variously referred to as pass-

Service in moving from the approach

ing bays, turnouts or partial climbing lanes and

segment to the grade, or

are typically 100 to 200 metres long. Because

Level of Service E existing on the grade.

vehicles entering the turnout do so at crawl speeds, the tapers can be very short, e.g. twen-

The Highway Capacity Manual warrant is

ty to thirty metres long, corresponding to taper

severe and does not, in any event, match the

rates of 1:6 to 1:10 in the case of 3,1 metre wide

speed reduction warrant. The provision of a

lanes.

climbing lane at a specific site is thus dependent on the type of warrant selected. It is according-

Climbing lanes usually have the same width as

ly suggested that, if a designer wishes to apply

the adjacent basic lane. In very broken terrain,

this warrant, the reduction considered should be

a reduction in width to as little as 3,1 metres can

of one or more Levels of Service, and not two or

be considered because of the low speeds of

more.

vehicles in the climbing lane.

On the same

grounds, the shoulder width may also be In view of the economic restraints on new con-

reduced but to not less than 1,5 metres. If the

struction, a compromise between convenience

shoulders elsewhere on the road are three

and cost effectiveness is required. The com-

metres wide, the additional construction width

promise proposed is that, while delay - seen as

required to accommodate the climbing lane and

a major criterion of Level of Service - is

reduced shoulder is thus only 1,6 metres.

employed in determination of the need for provi-


sion of a climbing lane, the delay considered

While the decision whether or not to provide the

would not be that suffered by the individual vehi-

climbing lane could be based on the economics

cle but rather by the entire traffic stream.

of the matter, the location of its terminals is

Commercially available software calculates the

dependent purely on safety based on the oper-

value of the time saved during the design life of

ational characteristics of truck traffic. The war-

the climbing lane and relates this to input from

rant for the provision of climbing lanes in terms

the designer in respect of construction and

of truck speed reduction is set at 20 km/h as

maintenance costs of the lane.

described previously. It is suggested, as a safety measure to allow for variation in the hill climb-

In mountainous terrain, where trucks are

ing capabilities of individual trucks, that, having

reduced to crawl speeds over extended dis-

established that a climbing lane is warranted, a

tances and relatively few opportunities for over-

speed reduction of 15 km/h be used to deter-

taking exist, the cost of construction of climbing

mine the location of the climbing lane terminals.

lanes may be prohibitive. Under these circum-

Table 4.10 shows critical lengths of grade in

stances, an alternative solution to the opera-

terms of a truck speed reduction of 15 km/h for

tional problem may be to construct short lengths

various gradients. It is recommended that the

of climbing lane as opposed to a continuous

full width of the climbing lane be provided at or

lane over the length of the grade. These short

before the end of the critical length, with this 4-36

length being measured on the preceding sag

The terminal may take one of two possible

curve from a point halfway between the Vertical

forms. One option is to provide a right-to-left

Point of Intersection (VPI) and the end of the

taper merging the basic lane with the climbing

vertical curve (EVC). The full width of the climb-

lane, followed by a left-to-right taper back to the

ing lane should be maintained until the point is

two-lane cross-section. The motivation for this

reached where truck speed has once again

layout is that faster-moving vehicles find it easy

increased to be 15 km/h less than the normal

to merge with slower-moving traffic. This is its

speed on a level grade.

only advantage. Should a vehicle not manage to complete the merge before the end of the

The length of the entrance taper should be such

right-to-left taper, its only refuge is the painted

that a vehicle can negotiate the reverse curve

island, thereafter being confronted by an oppos-

path with the benefit of a 2 per cent crossfall on

ing lane situation. Furthermore, in this layout,

the first curve followed by a negative superele-

the basic lane terminates at the end of the

vation of 2 per cent on the second curve with a

climbing lane with the climbing lane thereafter

short intervening tangent to allow for the rever-

becoming the basic lane. This is a contradiction

sal of curvature. Ideally, it should not be neces-

of the fundamental definitions of the basic and

sary for the vehicle path to encroach on the

auxiliary lanes. This option is not recommend-

shoulder. These conditions can be met by a

ed for two-lane roads, although it could possibly

taper which is about 100 metres long, corre-

be used on multilane or divided cross-sections.

sponding to a taper rate of 1 : 27.

This is

approximately half the taper rate applied to

The alternative is to provide a simple taper,

interchange off-ramps, which is appropriate as,

dropping the climbing lane after it has served its

in this case, the taper addresses a reverse path

purpose. A vehicle that cannot complete the

and not a single change of direction.

merging manoeuvre at the end of the climbing lane has the shoulder as an escape route. This is a safer option than that described above.

As stated above, the exit terminal of the climbing lane should be the point at which trucks

As described with regard to the entrance taper,

have accelerated back to a speed that is, at

a vehicle exiting from the climbing lane could

most, 15 km/h slower than the truck operating

negotiate a taper that is 100 metres in length.

speeds on the basic lanes. If there is a barrier

However, a flatter taper would allow time to find

line at this point, the lane should be extended to

a gap in the opposing traffic. A taper rate of 1:50

the point at which the barrier line ends. The rea-

is suggested for on-ramps at interchanges

son for this is that a vehicle entering the basic

where vehicles are required to negotiate only a

lane may inadvertently force an overtaking vehi-

single change of direction. The reverse curve

cle into the opposing lane. This is a potentially

path followed in exiting from an auxiliary lane

dangerous situation and the designer must

may require a still flatter taper rate for which rea-

ensure that there is sufficient sight distance to

son a rate of the order of 1:70 is suggested,

support appropriate decisions by the drivers

leading to a taper that is approximately 200

involved.

metres in length. 4-37


Passing lanes

there is an absence of passing opportunities. They are aimed at platoon dispersion and local

The procedure followed in the design of the

research has demonstrated that a passing lane

alignment of a road should seek, in the first

length of about one kilometre is adequate for

instance, to provide the maximum possible passing opportunity.

this purpose.

Thereafter, the need for

Numerous short passing lanes

are preferable to few long passing lanes and it is

climbing lanes should be evaluated and deci-

recommended that they be located at two, four

sions taken on where climbing lanes are to be

and eight kilometre spacings. Where traffic vol-

provided and what the lengths of these climbing

umes are low, the longest spacing can be used

lanes should be. At this stage, it is possible to

and, as traffic volumes increase, the intervening

relate the total passing opportunity to the overall

lanes can be added in a logical manner.

length of the road. With one-kilometre long passing lanes provided The analysis of two-lane roads as described in

at two-kilometre intervals, the next level of

the Highway Capacity Manual is based on two

upgrading would be a Two + One cross-section.

criteria, being percentage time spent following

In this case, the road is effectively provided with

and average travel speed. Both of these are

a three-lane cross-section from end to end with

adversely influenced by inadequate passing

the centre lane being alternately allocated to

opportunities. For example, a road with sixty

each of the opposing directions of flow. In keep-

per cent passing opportunity demonstrates a 19

ing with the spacings discussed above, the

per cent increase in time spent following by

switch in the direction of flow in the centre lane

comparison with a road with similar traffic vol-

should be at about two-kilometre intervals.

umes and unlimited passing opportunities. This


assumes a 50/50 directional split. The increase

Unlike climbing lanes, passing lanes tend to

in time spent following is greater with unbal-

operate at the speeds prevailing on the rest of

anced flows. If the flow is as low as 800 vehi-

the road. Reductions in lane width are thus not

cles per hour, passing opportunities limited to 60

recommended and passing lanes should have

per cent result in a reduction of the order of

the same width as the basic lanes.

three per cent in average speeds by comparison with the speeds on roads with unlimited passing

As recommended for climbing lanes, the

opportunity. It is accordingly necessary for the

entrance taper to a passing lane could be 100

designer to carry out an analysis based on the

metres in length and the length of the exit taper

Highway Capacity Manual to establish whether

double this to allow adequate time for merging

or not additional passing opportunities should

vehicles to find a gap in the through flow.

be provided in order to maintain the desired

Seeing that both the entrance and the exit

Level of Service.

tapers signal a change in operating conditions on the road, it is recommended that decision

Passing lanes are normally provided in areas

sight distance should be available at these

where construction costs are low and where

points.

4-38

High occupancy vehicle (HOV) lanes

in the cross-section. The essential point of difference lies in the fact that the passenger car is

As described above, auxiliary lanes are short

usually taken as the design vehicle for basic

and are intended only to deal with a specific cir-

lanes whereas the HOV lanes are designed to

cumstance.

accommodate buses. Kerb radii at intersections

As soon as this circumstance

changes, the auxiliary lane is dropped.

HOV

and the width of the turning lanes on bus routes

lanes, on the other hand, form part of the rapid

should be such that buses can negotiate these

transit system of a city and can thus be provid-

curves without encroaching on the adjacent

ed over a substantial distance. Whether they

lanes or, more importantly, on the sidewalks.

should be considered as auxiliary lanes could thus be debated.

Bus routes typically converge on the CBD. The HOV lanes, which, as part of the normal cross-

HOV lanes are typically applied on commuter

section, may have served well in the outlying or

routes with a view to encouraging the use of

suburban areas, could prove inadequate to

public transport or lift clubs hence reducing con-

accommodate the increased volume of bus traf-

gestion. The average occupancy of passenger

fic in the CBD. It may then be necessary to des-

cars is of the order of 1,5 persons per vehicle,

ignate various of the streets in the CBD as

whereas a municipal bus can convey 80 pas-

exclusive bus roads or, alternatively, to consider

sengers effectively replacing 50 or more pas-

a system similar to the O-Bahn routes.

senger cars in the traffic stream. In view of the fact that buses can be 2,6 metres wide, narrow

4.4.4

Kerbing

lane widths are inappropriate to HOV lanes that, ideally, should not be narrower than 3,6 metres,

Kerbs are raised or near-vertical elements that

i.e. allowing a clear space of 0,5 metres

are located adjacent to the travelled way and

between the sides of the vehicle and the lane

are usually used for:

markings.

• • •

HOV lanes have to be policed to ensure that only vehicles qualifying for the privilege use

Drainage control; Delineation of the pavement edge; and Reduction in maintenance operations by providing protection for the edge of

them. Signalisation can be employed to give

surfacing.

vehicles in HOV lanes priority over other road

Kerbing is normally only applied in urban areas

users. This is described in the South African

where vehicle speeds are relatively low.

Road Traffic Signs Manual in some detail. The combination of policing and priority usage

In rural areas, the drainage function is normally

underpins the effectiveness of HOV lanes but

accommodated by channels or open drains of

these operational issues are normally outside

various forms. Delineation is usually by means

the terms of reference of geometric design.

of an edge line or a contrasting colour on the It is important that the designer draws a distinc-

shoulder. Protection of the edge of surfacing

tion between the basic lanes and the HOV lanes

can be by means of buried edge blocks or, more 4-39


typically, by means of a thickened edge. A thick-

drainage. Perhaps their widest application is to

ened edge is simply a bitumen-filled V-shaped

be found in residential areas, where vehicles

groove cut into the base course.

can drive off the travelled way to park on the verge.

Kerbs may be barrier or semi-mountable or mountable. Barrier and semi mountable kerbs

Channels are usually about 300 mm wide, thus

normally are accompanied by a channel (or gut-

automatically providing an offset between the

ter) whereas the mountable kerb is, in effect, a

kerb and the edge of the travelled way. Where

channel itself.

channels are not provided, the offset should still be maintained for reasons of safety.

Barrier kerbs are intended primarily to control

4.4.5

drainage as well as access and can inhibit slow-

Shoulders

moving vehicles from leaving the roadway. When struck at high speeds, barrier kerbs can

Shoulders are the usable areas immediately

result in loss of control and damage to the vehi-

adjacent to the travelled way and are a critical

cle. In spite of the name, barrier kerbs are inad-

element of the roadway cross-section.

equate to prevent a vehicle from leaving the

provide:

road after a high-speed impact. In addition, a

• •

barrier kerb can lead to a high-speed errant vehicle vaulting over a barrier or guardrail. For

A recovery area for errant vehicles; A refuge for stopped or disabled vehicles;

•

this reason, barrier kerbing is not generally used on urban freeways and is considered undesir-


They

An area out of the travel lanes for emergency and maintenance vehicles; and

able on expressways and arterials with design

•

speeds higher than 70 km/h. Barrier kerbs are

In addition, shoulders support use of the road by

never used in conjunction with rigid concrete

other modes of transport, for example cyclists

barrier systems.

and pedestrians.

Semi-mountable kerbs have a face slope of 25

Regulation 298 of the regulations promulgated

mm/m to 62,5 mm/m and are considered mount-

in terms of National Road Traffic Act (Act

able under emergency conditions. They are typ-

93/1996) prohibits driving on the shoulder

ically used on urban freeways and arterials and

except that this is permitted:

also in intersections areas as a demarcation of

• • •

raised islands.

Lateral support of the roadway structure.

On a two-lane road; Between the hours of sunrise and sunset, While being overtaken by another vehicle.

Mountable kerbs have a relatively flat sloping

provided this can be done without endangering

face of 10 mm/m to 25 mm/m and can be

the vehicle, other vehicles, pedestrians or prop-

crossed easily by vehicles. They are particular-

erty and if persons and vehicles on the road are

ly useful as a form of lane demarcation on high-

clearly discernable at a distance of at least 150

speed roads but are not effective as a form of

metres. 4-40

Considering the above applications of the shoul-

Between the two extremes of 3,0 metres and

der, a stopped vehicle can be accommodated

1,0 metres, shoulder widths of 1,5 or 2,5 metres

on a shoulder that is three metres wide. There

could be used in the case of intermediate traffic

is no merit in adopting a shoulder width greater

volumes and speeds. These alternative shoul-

than this. The shoulder should, however, not be

der widths would not normally be used for the

so narrow that a stopped vehicle could cause

inner shoulders of a dual carriageway road.

congestion by forcing vehicles travelling in both

Table 4.16 illustrates the application of the vari-

directions into a single lane. A partly blocked

ous shoulder widths on undivided rural roads.

lane is acceptable under conditions of low speed and low traffic volume.

Assuming the

Paved widths of between 1,5 and 2,5 metres

narrowest width of lane, i.e. 3,1 metres, it would

should be avoided. The presence of the paving

be possible for two vehicles to pass each other

may tempt a driver to move onto the shoulder to

next to a stopped car if the shoulder were not

allow another vehicle to overtake, but these

less than 1,0 metres wide. Hazards, including

widths cannot accommodate a moving vehicle

the edges of high fills, cause a lateral shift of

with any safety.

vehicles if closer to the lane edge than 1,5 metres. Allowing for shoulder rounding of 0,5

These shoulder widths are recommended for

metres, the usable shoulder is thus 1,0 metres

adoption for new construction. In the case of

wide and this should be considered the irre-

rehabilitation or reconstruction projects, there

ducible minimum width of shoulder.

may not be sufficient width of cross-section to accommodate the desirable widths and some

Where the traffic situation demands a dual-car-

lesser width will have to be considered.

riageway cross-section, the greatest width of

As

pointed out in Section 4.4.2, it may be advisable

shoulder, i.e. three metres, is called for. This

to first reduce the shoulder width before consid-

width would apply to the outer shoulder. The

ering reductions of lane width.

inner shoulder need only be one metre wide:

•

to protect the integrity of the pavement-

The shoulder breakpoint is usually about 500

layers;

• •

mm beyond the edge of the usable shoulder to

to avoid drop-offs at the lane edge; and

allow for shoulder rounding.

provide space for roadmarkings

provided the median island is not kerbed, thus Where guardrails or other roadside appurte-

allowing a disabled vehicle to be moved clear of

nances have to be provided, these are located

the adjacent lane. If a barrier, such as kerbing

300 to 500 mm beyond the usable shoulder.

or a guardrail, makes the median island inac-

The shoulder breakpoint should be a further 500

cessible, the full shoulder width should be pro-

mm beyond these appurtenances, as a lesser

vided in the case of a six-lane cross section

distance will not provide the support needed by

because negotiating two lanes to reach the

a guardrail when hit by an errant vehicle.

safety of the outside shoulder (with a disabled vehicle) could be difficult. 4-41


The surfacing of shoulders is recommended:

should constitute adequate grounds for full sur-

• • •

facing of the shoulders.

For freeways; In front of guardrails; Where the total gradient, being the

Full surfacing implies continuous surfacing

resultant of the longitudinal gradient and

along the length of the road and not necessarily

the camber or superelevation, exceeds

across the full width of the shoulder, although

six per cent;

•

this is the desirable option.

Where the materials with which the

above that the minimum recommended width of

shoulders are constructed are readily

• • •

It is suggested

erodible, or where the availability of

shoulder is 1,0 metres. If it were considered

material for maintenance of the shoul-

necessary to surface the shoulder at all, there

ders is limited;

would be little or no operational advantage in

Where heavy vehicles would tend to use

surfacing a lesser width than this. In the case of

the shoulder as an auxiliary lane;

new construction, the designer has the option of

In mist belts; or

considering the economic merits of a relatively

Where significant usage by pedestrians

narrow surfaced shoulder vis-à-vis a wide

occurs.

unsurfaced shoulder. In the case of rehabilitation projects, it may be decided to retain the full


A patchwork of surfaced shoulders would be

3,0 metre shoulder but, as a cost-saving meas-

both unsightly and unsafe. Where the interven-

ure, to surface only half of the total width.

ing lengths of unsurfaced shoulders are short, it is suggested that they also be surfaced. As a

The cross fall on surfaced shoulders is normally

guideline, it is proposed that if surfacing sixty or

an extension of that on the travelled lanes.

more per cent of the shoulder is warranted, this

Where shoulders are not surfaced, the cross fall 4-42

is normally one per cent steeper than that on the

maximum stem thickness of 175 to 200 mm,

lane to allow for the rougher surface and the

corresponding to the diameter of a guardrail

consequently slower rate of flow of storm water

post, is recommended.

off the road surface. A further application of medians refers to access At night or during inclement weather it is impor-

management, where right turns into or out of

tant that the driver should be able to distinguish

local land uses are often discouraged on high-

clearly between the shoulder and the lane. This

speed roads.

can be accomplished by the use of a shoulder material of a contrasting colour or texture. Edge From the above it can be inferred that medians

marking is a convenient way of indicating the

are typically applied in the case of high speed or

boundary between the lane and the shoulder.

high volume roads with a basic function of

Rumble strips can also be used and have been

mobility.

shown to reduce the rate of run-off-road incidents by twenty per cent or more. Rumble strips can be raised or grooved.

Median islands can be as narrow as one metre,

Being intended to

provide the driver with an audible warning, the

which is sufficient to contain a median barrier

noise level they generate is unacceptable in

comprising back-to-back guardrails. This sug-

urban areas and should therefore only be used

gests that, including minimum width median

in rural areas.

shoulders, the minimum width of the median should be not less than three metres.

4.4.6

Inner

shoulders are often not provided in the urban

Medians

cross-section but kerbing would require an offThe median is the total width between the inner

set of about 300 mm. The minimum width of an

edges of the inside traffic lanes and includes the

urban median should thus be 1,6 metres.

central island and the median shoulders. As long ago as the early 1930s it was proposed to

Research has found that few out-of-control vehi-

"separate the up-traffic from the down-traffic", a

cles travel further than nine metres from the

function which the median fulfils to this day.

edge of lane, so that this width of median would

This separation is intended to reduce the proba-

be sufficient to avoid most head-on crashes.

bility of head-on crashes and also to reduce the nuisance of headlight glare (usually by the

In urban areas, medians often contain right-turn-

planting of shrubs on the central island). The

ing lanes.

reduction in head-on crashes is achieved

shoulder is invariably replaced by kerbing so

through selection of a suitable width of median

that the median would be the sum of the lane

or the use of median barriers. Shrubs can also

width plus the width required to provide a pedes-

serve as a barrier to prevent cross-median acci-

trian refuge. Pedestrians do not feel safe on

dents but the stems of the shrubs should not

median islands narrower than about two metres,

grow so thick as to become a further hazard. A

suggesting that the median should have a width 4-43

In intersection designs, the inner


of the order of 5,5 to 6,0 metres. This width is

piped drainage system is generally available.

adequate to accommodate pedestrians as well

The depressed median allows the roadbed to

as the right-turning lane.

If intersections are

drain into the median, specifically on curves

closely spaced, it may be necessary to apply

where water from the outer carriageway is pre-

this width to the full length of the median, where-

vented from draining across the inner carriage-

as with widely spaced intersections, e.g. 500

way. In the urban context, kerbing includes drop

metres or more between intersections, a lesser

inlets directing storm water into the underground

width can be applied between the intersections

system.

with the median being flared out by means of active tapers at the intersections.

Urban median islands are usually narrower than their rural counterparts and do not normally

Medians with a width of nine metres or more

have barriers. The barriers have to be terminat-

allow for individual grading of the two carriage-

ed at every intersection and at some entrances

ways, which can be useful in rolling terrain. In

so that the safety offered by the barrier is more

addition, these medians lend themselves to

than offset by the hazard of the barrier ends.

landscaping and to the creation of a park like

Kerbing offers a modest degree of protection to

environment.

Unfortunately, they create prob-

pedestrians who may be on the median while

lems at intersections by virtue of the long travel

crossing the road. In addition, kerbing can, to a

distances that they impose on turning vehicles.

limited extent, redirect errant vehicles back into

The incidence of crashes at intersections

their own lanes.

increases with increasing width of median and,


at widths of 20 metres, the intersection should

The speeds on rural roads make kerbing inap-

be designed as two intersections back-to-back,

propriate in this environment, as the driver of a

with traffic control on the roadway crossing the

vehicle striking a kerb at high speed would

median. The wide rural median does not trans-

almost certainly lose control of the vehicle, with

late well to the urban environment so that roads

this problem being compounded by the

on the outskirts of urban areas should be

inevitable damage to the front wheels of the

designed with medians appropriate to a future

vehicle.

urban characteristic. Depressed medians generally have flat slopes Medians may be either depressed or raised.

and a gently rounded bottom so that the driver

Depressed medians are normally used in rural

of a vehicle leaving the road has an opportunity

areas and raised medians in urban areas. This

to regain control, minimising occupant injury and

differentiation between rural and urban areas

vehicle damage. Overturning crashes are more

arises for two reasons: drainage and safety.

frequent on slopes steeper than 1 : 4 for median widths of six to twelve metres.

This should,

Storm water drainage in rural areas is generally

therefore, be considered the steepest allowable

above the surface for ease of maintenance

slope, with slopes of 1 : 6 or flatter being pre-

whereas, in urban areas, a well-developed

ferred. 4-44

4.4.7

Outer separators

At an intersection, the frontage road should either be terminated or moved a substantial dis-

The outer separator is the area between the

tance away from the through lanes.

This is

edges of the travelled way of the major road and

intended to safeguard the operation of the inter-

the adjacent parallel road or street. It compris-

section because vehicles attempting to turn

es the left shoulder of the major road, an island

from the through road to a frontage road could

and the right shoulder of the adjacent road or

very easily generate a queue that backs up onto

street. The outer separator serves as a buffer

the through lanes. Not only is this operationally

between through traffic and local traffic on a

undesirable but it could also be unsafe.

frontage or service road. It is typically applied where the corridor has to serve the two func-

Where it is anticipated that a road will have to be

tions of long distance travel and local accessi-

widened at some time in the future, t he width of

bility. An arterial passing through a local shop-

the outer separator should be such that it can

ping area is an example of this application.

accommodate the additional lane, hence minimising the extent of damage to the rest of the road cross-section.

If travel on the frontage road is one way and in the same direction as that on the adjacent

4.4.8

through lane, the outer separator can be as nar-

Boulevards

row as three metres or, if barriers are provided, Boulevards are only used in urban areas and

two metres.

are similar to outer separators with regard to At night, drivers on the through lane would find

their function and location. The principal differ-

an opposing direction of flow on the frontage

ence is that they separate a sidewalk and not a

road very confusing, being confronted by head-

frontage

lights both to the right and to the left. Under

Boulevards are a desirable feature because:

these circumstances, the width of the outer sep-

•

road

the

through

lanes.

The separation between the sidewalk and

arator should be substantially increased, prefer-

from

the

vehicular

traffic

provides

increased safety for pedestrians and

ably doubled, to minimise the effect of the

children at play;

approaching traffic, particularly headlight glare

•

on non-illuminated sections of the road.

The probability of a pedestrian/vehicle collision is reduced as the sidewalk is

Plantings or dazzle screens on the outer sepa-

placed some distance from the kerb;

•

rator are recommended for the same reason.

Pedestrians are less likely to be splashed by passing vehicles in wet weather;

On rural freeways, the outer separator should

•

be at least nine metres wide, based on the dis-

Space is provided for street furniture and

tance that an out-of-control vehicle is able to

streetscaping as well as for surface and

move away from the edge of the through lane.

underground utilities, and

•

Reference in the literature is to outer separator

Changes to the cross-slope of the side-

widths of twenty to thirty-five metres in rural

walk to provide for appropriate driveway

areas.

gradients are minimised using the 4-45


boulevard area to effect the gradient

have to be provided across boulevards, the vari-

change.

ation in slope should not be so drastic that vehicles cannot traverse the area without scraping

The verge, showing the location and dimensions

their undersides on the ridge between the

of the boulevard, is illustrated in Figure 4.11.

boulevard and the sidewalk.

Aesthetic considerations in the urban environBoulevards are as wide as the road reserve

ment are important, particularly when major

allows.

streets pass through or are adjacent to parkland and residential areas.

than two to three metres.

Desirably, the positive

aesthetic qualities of the adjacent land use are

4.4.9

carried over into the verge and boulevard areas of the street cross-section. As a feature of the

Bus stops and taxi lay-byes

Bus stops and lay-byes can be located within

urban landscape, boulevards are usually

the width of the boulevard. In this case, grass-

grassed or landscaped. If the boulevards are

ing of the boulevard is discontinued and the

narrower than 1,5 metres, they are surfaced

area surrounding the bus stop is paved as an

rather than grassed because of the mainte-

extension of the sidewalk to provide users of

nance difficulties associated with narrow strips.


Ideally, they should not be narrower

public transit with all-weather access to buses.

The entire area from the reserve boundary to

Pedestrian accidents often occur at bus stops.

the road edge is normally sloped towards the

This can be attributed to the fact that buses fre-

road to assist drainage, not only of this area but

quently stop too close to the road edge, thus

of the adjacent development as well. Because

obstructing oncoming drivers' view of pedestri-

Figure 4.11: Verge area indicating location of boulevard of the impedance offered by grass to overland

ans crossing the road.

flow, the slope of the boulevard should be at

stances, a pedestrian stepping out from behind

least four per cent. Local circumstances may

the bus would be moving directly into the path of

require steeper slopes but, where driveways

an oncoming vehicle. 4-46

Under these circum-

Two approaches can be adopted to minimise

The location of bus stops can have an adverse

this problem. If the bus stop is provided with

impact on safety. A bus at a stop located imme-

adequate entrance and exit tapers, it is easy for

diately in advance of intersections would force

buses to move well clear of the travelled way. If

left-turning vehicles into a situation of heavily

space permits, a painted island can be provided

reduced sight distance.

between the bus stop and the travelled way so

pulling out of the stop, the bus could seriously

that the stop is, in effect, a short length of auxil-

influence the operation of the intersection as a

iary lane. In addition to an approach aimed at

whole.

Furthermore, while

the physical dimensions of the bus stop, a further safety measure could be the provision of

Best practice suggests that bus stops should be

barriers preventing bus passengers from cross-

located beyond intersections.

ing the road until they have moved clear of the

should not be located more than about fifty

bus stop itself.

metres

from

the

nearest

However, they

intersection.

Figure 4.12: Typical layout of a bus stop A proposed typical layout of a bus stop is illus-

Destinations for bus passengers may be on the

trated in Figure 4.12. If the frequency of service

bus route itself but are more likely to be to one

on a particular road is high, e.g. where two or

side or the other of the route. The close prox-

more bus routes have converged upstream of

imity of the bus stop to an intersection offers

the bus stop, the length of the bus stop should

passengers a convenient route to their final des-

be increased to 25 metres to accommodate two

tination.

buses. If necessary, the tapers can be reduced

However, it is suggested that a bus

stop should not be located closer than about 15

to not less than 1 : 3 for design speeds of 70

metres from the kerb line of the intersecting

km/h or less.

road or street. A lesser spacing would make it

4-47


difficult for a left-turning bus to enter the bus

driveway entrances may have to have a steeper

stop and, furthermore, may result in encroach-

cross-slope than this to match the gradient of

ment on the sight triangle required by a driver on

the driveways but should not exceed a cross-

the intersecting road or street.

slope of five per cent.

4.4.10 Sidewalks

Kerbs, raised medians and channelising islands can be major obstructions to the elderly and

Pedestrian traffic is not encouraged in the road

people with disabilities, particularly those in

reserves of freeways, expressways or other

wheelchairs.

high-speed arterials and accommodation of

minimising the impact of these obstacles is to

pedestrian traffic is usually handled elsewhere.

provide ramps, also referred to as kerb cuts,

On all other urban streets, pedestrian traffic can

dropped kerbs or pram dips.

be expected and it is necessary to provide side-

have a slope of not more than about six per

walks.

cent. A kerb height of 150 millimetres would

The most common method for

Ramps should

thus require a ramp length of 2,5 metres. There should be a clear sidewalk width of 1,5 metres

In commercial areas or areas where the road

beyond the top end of the ramp so that, where a

reserve width is restricted, sidewalks may

ramp is provided, the overall sidewalk width

extend from the kerb to the road reserve bound-

should be not less than four metres.

ary. As discussed above, there is distinct merit

chairs may be 0,75 metres wide so that two

in placing a boulevard between the sidewalk

wheelchairs passing each other on the ramp

and the travelled way, but pedestrian volumes

would require a ramp width of the order of 2 to

may be so high that the entire available width

2,5 metres. If it is not possible to provide this

would have to be utilised for the sidewalk. The

width, a width of not less than 1,5 metres should

width of a sidewalk should not be less than 1,5

be considered.

metres and a minimum width of two metres should be provided near hospitals and old-age

The designer should be aware that, in accom-

homes where wheelchair traffic could be expect-

modating one group of pedestrians with disabil-

ed. If the sidewalk is immediately adjacent to


Wheel

ities, a different group might be disadvantaged

the kerbing, the minimum width should be

in the process.

increased by about 0,6 to 1 metre. This is to

Visually impaired pedestrians

would have trouble in locating the kerb face in

make provision for fire hydrants, street lighting

the presence of a ramp.

and other road furniture. It also allows for the

As a vertical face

across the sidewalk would be unexpected, even

proximity of moving vehicles and the opening of

by sighted pedestrians, the sides of the ramp

car doors.

should also be sloped.

The normal cross-slope on a sidewalk is 2 per

Sidewalks are not normally provided in rural

cent. Cross-slopes steeper than this present a

areas. It should, however, be noted that approx-

problem to people with walking impairments or

imately half the fatalities on the South African

who are in wheel chairs. Sidewalks crossing

road network are pedestrians, with many of 4-48

these

fatalities

occurring

in

rural

areas.

pacted regularly to provide pedestrians with a

Provision should therefore be made for pedes-

hard surface to walk on. In high rainfall areas, a

trian safety outside urban areas. Paved foot-

portion at least of the shoulder should be paved,

ways could be considered under the warranting

with this paved area being at least 1,5 metres

conditions listed in Table 4.17.

wide. Furthermore, the road shoulder should be well drained to prevent the accumulation of

Footways can be as little as one metre wide, but

water, which would force pedestrians to walk on

a width of 1,8 metres would allow two people to

the carriageway.

walk side by side.

4.4.11 Cycle paths The safest location for footways is at the edge of the road reserve. This location is not popular

Changes from single to multiple land usage will

with pedestrians because the footway then fol-

result in shorter trip lengths, making the bicycle

lows all the variations in the natural ground

a more popular form of transport. In addition,

level. In rolling or mountainous terrain through

people are becoming conscious of the need for

cuts and fills, such a footway would not make for

exercise and of the bicycle as means of exer-

comfortable walking. Even if this footway were

cise. Finally, unlike the motor vehicle, bicycles

provided, pedestrians would almost certainly

are environmentally friendly.

prefer to walk on the more level surface of the

If adequately

planned, designed and maintained, cycle paths

shoulder. In level terrain, the footway should, if

can play an important role in the transportation

possible, be situated at least three metres away from the travelled way. This corresponds to a

system. It is important to realise that cyclists

location immediately outside the edge of the

need sufficient space to operate with safety and

usable shoulder in the case of a high volume

convenience rather than simply being assigned

high-speed road.

whatever space is left over after the needs of

In cases where footways are not warranted but

vehicular traffic has been accommodated.

where a large number of pedestrians walk alongside the road, the road shoulder should be

The basic requirements of cyclists are:

upgraded to cater for them. The minimum width

• • •

of these shoulders should be three metres. If not surfaced, they should be bladed and com-

4-49

Space to ride; A smooth surface; Speed maintenance, and


•

safely. The surface of a cycle path should not

Connectivity.

deviate from a three-metre straightedge by The bicycle design envelope and clearances are

more than 5 mm and should also be shaped to

illustrated in Figure 4.13. The one metre enve-

existing features to within 5 mm.

lope allows for the width of the bicycle as well as Adequate clearances to

For bicycles to be effective as a means of trans-

fixed objects and passing vehicles should be

portation, cyclists must be able easily to main-

provided, in addition to the one metre envelope.

tain a steady speed with ease. Cyclists typical-

for erratic tracking.

ly travel at speeds of twenty to thirty km/h some-


Bicycles have narrow tyres inflated to high pres-

times reaching 50 km/h on downgrades. Once

sures and have no suspension system to speak

slowed or stopped it takes considerable time

of. A smooth surface is therefore desirable for

and effort to regain the desired operating speed.

bicycles to be used effectively, comfortably and

Bicycle routes should thus be designed for con-

Figure 4.13: Bicycle envelope and clearances

4-50

tinuous movement, avoiding steep gradients,

from the roadway and from which all

rough surfaces, sharp corners, intersections or

motor traffic, with the exception of main-

the need to give way to other road users.

tenance vehicles, is excluded. Cycle paths may be located within the road reserve or in an independent reserve.

It should be possible to undertake and complete journeys by bicycle.

Bicycle routes on roadCycle lanes should have the widths indicated in

ways or separate paths should form a connect-

Table 4.18.

ed network on which bicycle trips can be made effectively and conveniently. Connectivity is an

The geometry dictated by motor vehicles is gen-

important aspect of effective bicycle routes and

erally adequate for bicycles except that bicycles

should be given careful consideration during the

have a longer stopping sight distance.

planning process. In the case of separate cycle paths, the bicycle Facilities for bicycles can take the form of a:

becomes the design vehicle, with dimensions,

•

Shared roadway/cycle lane where motor

performance, stopping sight distance, minimum

vehicles and bicycles travel in a com-

horizontal and vertical curvature, and clear-

mon lane;

•

ances.

Cycle lane, which is part of the travelled way but demarcated as a separate lane;

•

4.4.12 Slopes

Shoulder lane, which is a smooth, paved portion of the shoulder, properly demar-

The slopes of the sides of the road prism are,

cated by pavement markings or traffic

like those of medians, dictated by two different

signs. As the shoulder provides a useful

conditions.

area for cycling with few conflicts with

safety and a slope of 1:4 is the steepest accept-

fast-moving motor vehicles, this facility

•

Shallow slopes are required for

is very useful in rural areas.

able slope for this purpose. The alternative is to

Cycle path, which is physically removed

accept a steeper slope and provide for safety by 4-51


some other means, such as barriers.

In this

to provide the road and its appurtenant works.

case the steepest slope that can be used is dic-

Utilities not directly connected with the road,

tated by the natural angle of repose and erodi-

e.g. telephone or power lines are normally locat-

bility of the construction material.

ed in the verge.

Non-cohesive materials require a slope of 1:2,

In urban areas, the area between the edge of

whereas cohesive soft materials can maintain a

the travelled way and the road reserve bound-

slope of 1:1,5. Cuts in firm cohesive materials

ary provides space for a variety of elements

such as stiffer clays can be built to a slope of

that, for convenience, are summarised in Table

1:1. Rock cuts can be constructed to a slope of

4.19.

1:0,25 (4:1) provided that the material is reasonably unfissured and stable.

Some of these elements have been discussed previously.

The intention is to provide the

It is stressed that the slopes suggested are only

designer with a checklist of elements that should

an indication of normally used values.

The

be accommodated.

Some elements will be

detailed design of a project should therefore

mutually exclusive.

For example, the use of

include geotechnical analysis, which will indi-

barrier kerbs indicates that mountable kerbs

cate the steepest slopes appropriate to the con-

cannot be present.

struction or in-situ material. Economic analysis

temporary change in cross-section, for example

will indicate the height of fill above which a slope

the boulevard being replaced by a bus embay-

of 1:4 should be replaced by a steeper slope

ment. Yet others may overlap, for example the

and alternative provision made for safety. As a

driveway approach that crosses a sidewalk.

Others may represent a

rule of thumb, the transition from the flat slopes to slopes dictated by the materials typically

Elements that are most likely to be accommo-

occurs at a fill height of about 3 m.

dated in the verge are;

•

Berms intended to function as barriers protecting the surrounding development

4.4.13 Verges

from visual intrusion or noise;


• • •

The verge is defined as the area between the longitudinal works and the road reserve bound-

Cut and fill slopes; Driveway approaches, and Underground services.

ary. The limit of the longitudinal works in the case of the rural cross-section is often at the top

Even in the unlikely event that none of these

of cut or the toe of fill. Where drainage works,

elements have to be provided, there has to be a

such as side drains or catch water drains, are

clear space between the edge of the travelled

required, these form part of the longitudinal

way and the road reserve boundary. This space

works, which may thus be wider than the actual

would provide sight distance in the case of hor-

road prism. In rural areas the verge simply rep-

izontal curvature and also allow for emergency

resents the difference in width between a statu-

stopping. Furthermore, there should be some

tory road reserve and the width actually needed

flexibility to accommodate future unknowns. It 4-52

is suggested that this clear space should ideally

road itself. However, to the driver the presence

be a minimum of five metres wide with an

of a pole is a hazard to be avoided and whether

absolute minimum width of three metres.

the pole is carrying a power line or a streetlight is a matter of indifference. Given this broader

4.4.14 Clearance profiles

approach to utilities, surface utilities typically located in the reserve include:

The clearance profile describes the space that is

• • • • •

exclusively reserved for provision of the road. It defines the lowest permissible height of the soffit of any structure passing over the road and also the closest approach of any lateral obstacle to the road cross-section.

Electrical transmission lines; Telephone lines; Street lighting; Traffic signal poles, and Fire hydrants.

Clearance profiles Underground utilities include:

are described in detail in Chapter 10.

• • • • •

4.4.15 Provision for utilities

Both surface and underground utilities are often

Storm and foul water sewers; Water reticulation; Buried telephone lines; Gas pipelines, and Power transmission cables.

located within the road reserve. Utilities convey the sense of services not directly related to the

Most urban authorities have guidelines for the

4-53


Figure 4.14: Collision rate


Figure 4.15: Prediction of utility pole crashes placement of utilities. The use of an integrated

utilities is by manholes and open manholes are

process in the planning and location of road-

an unnecessary hazard to pedestrians. In older

ways and utilities is encouraged in order to

municipal areas, services were sometimes

avoid or at least to minimise conflicts.

located under the roadway itself. This practice should be actively discouraged as it places both

As a rule, underground utilities should be locat-

maintenance workers and passing vehicles at

ed in the verges or boulevards. Access to these

risk. Every time the road is resurfaced, it is nec4-54

4.4.16 Drainage elements

essary to remove the manholes and replace them at the new level. This operation carries an element of risk but, if not carried out, the man-

The process of design of storm water drainage

hole is at a lower level than the road surface and

systems is exhaustively discussed in the South

the drop could be sufficient for a driver to lose

African Roads Agency Drainage Manual, 1986.

control of the vehicle. The lower level of the manhole would, during rainy weather, lead to

In this section, discussion is limited to the ele-

the creation of a pond of water that could slow-

ments that the designer should incorporate in

ly drain into the conduits of the buried utility,

the cross-section to ensure adequate drainage

possibly leading to disruption of the service

of the road reserve and the adjacent land.

being provided. A distinction is drawn between rural and urban drainage. Rural drainage focuses largely on the The problem with surface utilities is that they are

swift removal of storm water from the travelled

carried on poles that can be hit by errant vehi-

way onto the verge and on its movement to a

cles. Research has indicated that the frequen-

point where it can be taken from the upstream to

cy of crashes is a function of the pole density in

the downstream side of the road. In an urban

poles per kilometre and the average pole offset

environment, the road reserve serves as the

from the travelled way. The crash frequency is

principal conduit of storm water from surround-

typically of the order of 0,1 crashes per kilome-

ing properties and its conveyance to a point

tre per year with a pole spacing of less than 20

where it can be discharged into natural water-

poles per kilometre and an offset of eight

courses.

metres. When the pole density is higher than 30

Rural drainage is, in short, the

removal of water from the road reserve whereas

poles per kilometre and the offset less than one

urban drainage attracts water to the reserve.

metre, the collision rate climbs to a high of 1,5 crashes per kilometre per year. This is illustrat-

In both rural and urban environments, storm

ed in Figure 4.14.

water drainage is aimed both at the safety of the road user and the integrity of the design layers

A nomograph predicting utility pole crashes is

of the road.

given in Figure 4.15.

exclusively directed towards safeguarding the

Previously, rural drainage was

design layers. It was previously common pracThe example shows that a road with an ADT of

tice to recommend a minimum depth of drain.

11 000 vehicles and a pole density of 40 poles

The safety of the road user dictates rather that a

per kilometre will experience 0,75 crashes per

maximum depth of drain be specified. The rec-

kilometre per year if the pole offset is 1,5

ommended maximum depth is 500 mm.

metres. If the designer were to increase the

volume of water to be conducted by a drain thus

pole offset to 3 metres, the crash rate would

indicates the required width of the drain rather

reduce to 0,5 crashes per kilometre per year, an

than its depth, since the need to keep the design

improvement of 33 per cent.

layers unsaturated has not changed.

4-55

The


Rural drainage

scour is likely to occur are given in Table 4.20. Conventional open-channel hydraulics will, in

Rural drainage is normally by means of unpaved

conjunction with Table 4.20, indicate when

open drains, which may either silt or scour,

either silting or scouring is likely, and hence

depending on the flow speeds in them. Both

whether it is necessary to pave a drain or not.

silting and scouring of a drain increase the hazard to the road user. Scour would lead to the

As a rough guide to longitudinal slopes, it is sug-

creation of a deep channel that would be impos-

gested that unpaved drains should not be steeper

sible to traverse with any degree of safety. It

than 2 per cent, or flatter than 0,5 per cent.

may also cause erosion of the shoulder and ulti-

Paved drains should not be flatter than 0,3 per

mately threaten the integrity of the travelled way

cent. Practical experience indicates that it is dif-

itself. Silting may block the drain, so that water

ficult to construct a paved drain accurately to the

that should have been removed would be dis-

tolerances demanded by a slope flatter than 0,3

charged onto the road surface.

per cent, so that local imperfections may cause silting of an otherwise adequate drain.

The effectiveness of the drain depends on water speed, which is a function of longitudinal slope,

Where the longitudinal slope is so flat that self-

as well as of other variables. There is a range

cleansing water speeds are not achieved, even

of slopes over which the flow velocity of water

with paving, it will be necessary to consider a

on in-situ materials will be so low that silting

piped drainage system.

occurs, and another range where the flow velocOn

As an alternative to lining a material subject to

slopes between these two ranges neither silting

scour, it is possible to reduce flow velocity by

nor scouring will occur and unpaved drains will

constructing weirs across an unpaved drain.

be effective.

The drain will then in effect become a series of

ity will be high enough to cause scour.

stilling basins at consecutively lower levels.


Paving solves some of the problems caused by

While this could be an economical solution in

both silting and scouring. Paving generally has

terms of construction cost, it has the disadvan-

a lower coefficient of roughness than in-situ

tage that an area of deep localised erosion,

materials, so that water speeds are higher in a

immediately followed by a stone-pitched or con-

paved drain than in an unpaved drain with the

crete wall, would confront an errant vehicle. If

same slope. Furthermore, it is possible to force

this alternative is to be considered at all, it

higher speeds in the paved drain by selection of

should be restricted to roads with very low traf-

the channel cross- section. The problem of silt-

fic volumes and the weirs should be spaced as

ing can be resolved, at least partially, by paving

far apart as possible.

the drain. Drains constructed through in-situ materials The flow velocity below which silting is likely to

generally have flat inverts so that, for a given

occur is 0,6 m/s. Flow velocities above which

flow, the flow velocity will be reduced. The flat 4-56

inverts reduce the possibility of scour and are

reduce the likelihood of a vehicle digging its

easy to clear if silting occurs. Paved drains, not

front bumper into the far side of the drain and

being susceptible to scour, have a V-profile.

somersaulting.

Self-cleansing velocities are thus achieved at relatively small flows and the need for mainte-

Typical drain profiles are illustrated in Figure 4.16.

nance is reduced.

The sides of the drain should not be so steep as

(a)

Side drains

to be dangerous to the road user; a maximum slope of 1:4 is recommended.

Side drains are located beyond the shoulder

Ideally, both

breakpoint and parallel to the centre line of the

sides of the drain should be designed to this

road. While usually employed in cuts, they may

slope or flatter. Where space for the provision of

also be used to run water along the toe of a fill

the drain is restricted, the slope closest to the

to a point where the water can conveniently be

road should remain at 1:4 and the outer slope

diverted, either away from the road prism or

steepened. This has the effect of positioning the

through it, by means of a culvert.

drain as far as possible from the path of vehi-

in conjunction with fills, side drains should be

cles. One example of this is a side drain in a

located as close to the edge of the reserve

cut, where the outer slope of the side drain forms an extension of the cut face.

When used

boundary as is practicable to ensure that ero-

These

sion of the toe of the fill does not occur. Side

slopes, in combination with the flat invert, give

drains are intended as collectors of water and

the trapezoidal profile of an unpaved drain.

the area that they drain usually includes a cut face and the road surface.

It is recommended that the bottom of a lined Vprofile and the junctions between the sides and

(b)

Edge drains

bottom of an unlined trapezoidal profile be slightly rounded.

The rounding will ease the

Edge drains are intended to divert water from fill

path of an errant vehicle across the drain, and

slopes that may otherwise erode either because 4-57


Figure 4.16: Typical drain profiles


of the erodibility of the material or because they

located almost under a guardrail would heighten

are subjected to concentrations of water and

the possibility that a vehicle wheel might snag

high flow velocities.

under the guardrail.

Guardrail posts tend to

serve as points of concentration of water, so that, as a general rule, edge drains are warrant-

Edge drains are constructed of either concrete

ed when the fill material is erodible or when

or premixed asphalt.

guardrails are to be installed.

have a height of 75 to 80 mm, and are trape-

Premix berms normally

zoidal in profile with a base width of 250 mm and Edge drains should preferably be raised rather

a top width of 100 mm. Concrete edge drains

than depressed in profile. A depressed drain

are normal barrier kerbs and channels. These 4-58

require a properly compacted backing for stabil-

used, a transverse slope flatter than 1:10 may

ity and are, therefore, not as easy to construct

make it difficult to protect the design layers of

as premix berms.

the road.

Unlike side drains, median drains,

whether lined or not, are generally constructed

(c)

Catch water drains

with a shallow V-profile with the bottom gently rounded.

The catch water drain, a berm located at the top of a cut, is to the cut face what the edge drain is

(e)

Chutes

to a fill. It is intended to deflect overland flow from the area outside the road reserve away

Chutes are intended to convey a concentration

from the cut face. Even if the cut is through

of water down a slope that, without such protec-

material which is not likely to scour, the catch

tion, would be subject to scour. They may vary

water drain serve to reduce the volume of water

in size from large structures to half-round pre-

that would otherwise have to be removed by the

cast concrete products, but they are all open

side drain located at the bottom of the cut face.

channels. Flow velocities are high, so that stilling basins are required if down-stream erosion

Catch water drains are seldom, if ever, lined.

is to be avoided. An example of the application

They are constructed with the undisturbed top-

of chutes is the discharge of water down a fill

soil of the area as their inverts and can readily

slope from an edge drain.

be grassed as a protection against scour.

chutes require attention to ensure that water is

Transverse weirs can also be constructed to

deflected from the edge drain into the chute,

reduce flow velocities, since the restrictions pre-

particularly where the road is on a steep grade.

viously mentioned in relation to weirs do not

It is important that chutes be adequately spaced

apply to catch water drains. The cut face and

to remove excess water from the shoulders of

the profile of the drain reduce the probability of

the road. Furthermore, the dimensions of the

a vehicle entering the drain but, should this hap-

chutes and stilling basins should be such that

pen, the speed of the vehicle will probably be

these drainage elements do not represent an

low.

excessive risk to errant vehicles.

The entrances to

Generally,

they should be as shallow as is compatible with

(d)

their function and depths in excess of 150 mm

Median drains

should be viewed with caution. Median drains do not only drain the median but also, in the case of a horizontal curve, prevent

Because of the suggested shallow depth of

water from the higher carriageway flowing in a

chutes, particular attention should be paid to

sheet across the lower carriageway. The space

their design and construction to ensure that the

available for the provision of median drains

highly energised stream is not deflected out of

makes it possible to recommend that the trans-

the chute. This is a serious erosion hazard that

verse slopes should be in the range of 1:4 to

can be obviated by replacing the chute with a

1:10. If the narrowest median recommended is

pipe. 4-59


Ch-4(Road Design Elements)

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