Highway Engineering

Combining Equations 6.4 and 6.5 with the appropriate design speeds and round-

ing the resulting values as appropriate yields the set of stopping sight distances

given in Table 6.7.

6.4.3 Overtaking sight distance

Overtaking sight distance is of central importance to the efficient working of a

given section of highway. Overtaking sight distance only applies to single car-

riageways. There is no full overtaking sight distance (FOSD) for a highway with

a design speed of 120 km/hr since this design speed is not suitable for a single

carriageway road.

Full overtaking sight distances are much larger in value than stopping sight

distances. Therefore, economic realities dictate that they can only be complied

with in relatively flat terrain where alignments, both vertical and horizontal,

allow the design of a relatively straight and level highway.

Values for different design speeds are given in Table 6.8.

Geometric Alignment and Design 165

Design speed (km/hr)

120 100 85 70 60 50

Full overtaking sight distance (m) — 580 490 410 345 290

Table 6.8Full overtaking sight distances for different design speeds (source: TD 9/93

(DOT, 1993))

Full overtaking sight distance is measured from vehicle to vehicle (the hazard

or object in this case is another car) between points 1.05 m and 2.00 m above

the centre of the carriageway. The resulting envelope of visibility for this set of

circumstances is shown in Fig. 6.9.

Full overtaking si ght distance

2.0 m 2.0 m

1.05 m

Envelope of visibility

1.05 m

Figure 6.9Measurement of full overtaking sight distance (FOSD).

Full overtaking sight distance is made up of three components: d 1 ,d 2 and d 3.

d 1 =Distance travelled by the vehicle in question while driver in the overtaking

vehicle completes the passing manoeuvre (Overtaking Time)