Highway Engineering

The proper geometric design of a highway ensures that drivers use the facil-

ity with safety and comfort. The process achieves this by selecting appropriate

vertical and horizontal curvature along with physical features of the road such

as sight distances and superelevation. The ultimate aim of the procedure is a

highway that is both justifiable in economic terms and appropriate to the local

environment.

6.2 Design speed, stopping and overtaking sight distances,

6.2.1 Introduction

The concept of ‘design speed’ lies at the centre of this process. The design speed

of a highway serves as a guide in the selection of the physical features referred

to above. Selection of the correct design speed ensures that issues of both safety

and economy in the design process are addressed. The chosen design speed

must be consistent with the anticipated vehicle speeds on the highway under

consideration.

The standard design speeds are 50 km/hr, 60 km/hr, 70 km/hr, 85 km/hr, 100

km/hr and 120 km/hr. These bands are based on the premise that it is consid-

ered acceptable if 85% of drivers travel at or below the designated design speed

for a given highway, generally inducing a situation where approximately 99% of

Geometric Alignment and Design 155

Centre

line

7.30m carriageway Central reserve

Hardstrip

1.00

Hardstrip

1.00

Hardstrip

3.65m 3.65m 1.00

Figure 6.3Dual 2-lane all-purpose (D2AP).

Hardstrip

1.00

Central reserve

Hardstrip

1.00

11.00m carriageway

Lane

line

Hardstrip

1.00

Lane

line

3.65m 3.70m 3.65m

Figure 6.4Dual 3-lane all-purpose (D3AP).