Highway Engineering

6.6 Vertical alignment,

6.6.1 General

Once the horizontal alignment has been determined, the vertical alignment of

the section of highway in question can be addressed. Again, the vertical align-

ment is composed of a series of straight-line gradients connected by curves, nor-

mally parabolic in form (see Fig. 6.17). These vertical parabolic curves must

therefore be provided at all changes in gradient. The curvature will be deter-

mined by the design speed, being sufficient to provide adequate driver comfort

with appropriate stopping sight distances provided.

The desirable maximum vertical gradients are shown in Table 6.11.

178 Highway Engineering

IP

IP

Uphill straight-

line gradient

(+ve)

Crest curve

(parabola)

Downhill

straight-line

gradient (-ve)

Sag curve

(parabola)

Uphill straight-

line gradient

(+ve)

Figure 6.17Example of typical vertical alignment.

Road type Desirable maximum

gradient (%)

Motorway 3

All-purpose dual carriageway 4

All-purpose single carriageway 6

Table 6.11Desirable

maximum vertical

gradients

In difficult terrain, use of gradients steeper than those given in Table 6.11 may

result in significant construction and/or environmental savings. The absolute

maximum for motorways is 4%. This threshold rises to 8% for all-purpose roads,

with any value above this considered a departure from standards (DoT, 1993).

A minimum longitudinal gradient of 0.5% should be maintained where possi-