Highway Engineering

6.3 Geometric parameters dependent on design speed,

For given design speeds, designers aim to achieve at least the desirable minimum

values for stopping sight distance, horizontal curvature and vertical crest curves.

However, there are circumstances where the strict application of desirable

minima would lead to disproportionately high construction costs or environ-

mental impact. In such situations either of two lower tiers can be employed:

 Relaxations


 Departures.

Relaxations

This second tier of values will produce a level of service that may remain accept-

able and will lead to a situation where a highway may not become unsafe. The

limit for relaxations is defined by a set number of design speed steps below a

benchmark level – usually the desirable minimum (TD 9/93). Relaxations can

162 Highway Engineering

Example 6.2 Contd

Determine the 85th percentile speed and compare it with the derived design

speed.

Solution

Speed range (km/hr) Observed cars Percentile speed

with speed within

or below this range

Less than 60 15 3rd

60–64 25 4th

65–69 41 7th

70–74 142 25th

75–79 282 50th

80–84 478 85th

85–89 540 96th

90–94 555 98th

95–99 561 99th

Greater than 100 562 100th

Table 6.6Percentiles for

observed speed ranges

From the figures in Table 6.6 it can be seen that the 85th percentile speed

is in the range of 80–84 km/hr. Thus the observed driver speeds are appre-

ciably below the design speed/mandatory speed limit that has been allowed

for and, as a consequence, the road would not be a priority for upgrading/

improvement.