## Sight distance - geometric design of roads, Civil Engineering

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Sight Distance:

Sight distance is the distance along a road at which a driver has a visibility of objects and can safely stop his vehicle or overtake another vehicle.

Stopping Sight Distance

The stopping sight distance is the distance over which the driver traveling at a speed can apply brakes and bring the vehicle to a stop. It is composed of two component:

(a) Distance (d1) travelled in the brief time interval between the sighting of an object on the road and his applying the brake, known as the perception and brake reaction time.

(b) Braking distance (d2).

If t is the perception and brake reaction time in seconds, and v is the speed in m/sec,

d1 = vt

= 0.278 Vt, if V = speed in Km/hr t is taken as 2.5 seconds. Thus,

d1 = 0.278 × V × 2.5

= 0.695 V

The braking distance d2 if given by :

v2 = 2. a. d2, where a = deceleration = f . g,

where f = coefficient of friction, which varies from 0.3 to 1.0

g = acceleration due to gravity = 9.81 m/sec2

∴          v2 = 2 × f × 9.81 × d2

∴          d2 =     v2/2 ×  f × 9.81= (V/3.6)2  (1/2 × f × 9.81 ) =V2/254 f

where V = speed in Km/hr.

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