Highway Engineering

the drivers travel at or below one speed category above the design speed (i.e. if

the speed limit is set at 85 km/hr, it can be assumed that 85% of the drivers will

travel at or below this value while 99% will travel at or below 120 km/hr).

The speed bands are thus related to each other by a factor equal to the fourth

root of 2, taken as being approximately 1.19. Thus, if the chosen design speed

is by definition the 85th percentile speed for that highway, then the next speed

band up will constitute its 99th percentile speed. The same factor separates the

chosen design speed and the next speed band down, which constitutes the 50th

percentile or mean speed, thus:

The design bands can thus be structured as shown in Table 6.2.

99 2 1 19

802119

4

4

th percentile 85th percentile

5th percentile 5 th percentile

∏==

∏==

.

.

156 Highway Engineering

85th percentile speed 99th percentile speed

120 145

100 120

85 100

70 85

60 70

Table 6.2Framework

for design speeds

The geometric properties associated with the design speed of a highway con-

stitute ‘desirable values’ at which 85% of the drivers are travelling with complete

safety. The geometric values of the next design speed up constitute a standard

at which 99% of the drivers can travel safely at the original design speed. Con-

versely, the geometric values of the next design speed down will constitute a

relaxation at which only 50% of the drivers will be in a position to travel with

complete safety at the original design speed. Such values constitute absolute

minimum values. However, these may have to be adopted in difficult design con-

ditions where many constraints both physical and otherwise exist.

Thus, in conclusion, the 85th percentile speed is selected as the design speed

on the basis that it constitutes the most appropriate and judicious choice, as use

of the 99th percentile would prove extremely expensive while extensive use of

the 50th percentile may prove unduly unsafe for the faster travelling vehicles.

6.2.2 Urban roads

Within the UK the design speed for an urban highway is chosen on the basis of

its speed limit. The value chosen will allow a small margin for speeds greater

than the posted speed limit (DoT, 1993). For speed limits of 48, 64, 80 and

96 km/hr, design speeds of 60B, 70A, 85A and 100A respectively are employed.