Highway Engineering

vast majority (>95%) of driver heights will be greater than 1.05 m while, at the

upper range, 2 m is set as the typical eye height for the driver of a large heavy

goods vehicle. With regard to object heights, the range 0.26 m to 2 m is taken as

encompassing all potential hazards on the road.

Checks should be carried out in both the horizontal and vertical planes. This

required envelope of visibility is shown in Fig. 6.8.

164 Highway Engineering

2.0 m

0.26 m

2.0 m

1.05 m

Envelope of visibility

Stopping sight distance

Figure 6.8Measurement of stopping sight distance.

The distance itself can be subdivided into three constituent parts:

 The perception distance – length of highway travelled while driver perceives

hazard


 The reaction distance – length of highway travelled during the period of time

taken by the driver to apply the brakes and for the brakes to function


 The braking distance – length of highway travelled while the vehicle actu-

ally comes to a halt.

The combined perception and reaction time,t, can vary widely depending on

the driver. However, in the UK, a value of 2 seconds is taken as being appro-

priate for safe and comfortable design.

The length of highway travelled during the perception-reaction time is calcu-

lated from the formula:

Perception-reaction distance (m) =0.278tV (6.4)

where

V=initial speed (km/hr)

t =combined perception and reaction time (s)

A rate of deceleration of 0.25g is generally used for highway design in the UK.

This value can be achieved on normally-textured surfaces in wet conditions

without causing discomfort to the driver and passengers.

Braking distance (m) =v^2 /2w (6.5)

where

v=initial speed (m/s)

w=rate of deceleration (m/s^2 )