Highway Engineering

6.6.5 Crossfalls

To ensure adequate rainfall run-off from the surface of the highway, a minimum

crossfall of 2.5% is advised, either in the form of a straight camber extending

from one edge of the carriageway to the other or as one sloped from the centre

of the carriageway towards both edges (see Fig. 6.19).

6.6.6 Vertical crest curve design and sight distance requirements

In the case of a crest curve, the intervening highway pavement obstructs the vis-

ibility between driver and object. The curvature of crest curves should be suffi-

ciently large in order to provide adequate sight distance for the driver. In order

to provide this sight distance, the curve length Lis a critical parameter. Too

great a length is costly to the developer while too short a length compromises

critical concerns such as safety and vertical clearance to structures.

For vertical crest curves, the relevant parameters are:

 The sight distance S


 The length of the curve L

Geometric Alignment and Design 183

Example 6.7 Contd

Using Equation 6.46:

Co-ordinates of highest point on crest curve:

Using Equations 6.47 and 6.48

Since, from Equation 6.44

p=(y +Y) ∏ x

Y =px -y

=0.04(120) -2.4

=2.4 m

x

p

pq

210 0.04 0.04 0.03

120 m

y p

2p q

210 0.04 2 0.04

2.4 m

(^22)

=

-

=¥()∏+()

=

=

()-

=¥()∏¥()+

=

L

L 003.

eqp

8

0.03 0.04 210 8

8375 m

=- -()=--()-()¥ ∏

=

L

1.