It is the length of the road ahead visible to a driver at any instance. IRC (Indian Roads congress) has standardized the definition by assuming the eye level of driver as 1.2 m above the road surface and height of obstruction 0.15 m from the road surface.
These are the three main types of sight distances, that are considered while the design of the roads:
(a) Stopping Sight Distance
(b) Overtaking Sight Distance
(c) Sight Distance at the Intersections
(a) Stopping Sight Distance: Stopping Sight Distance is the sight distance required for a driver to effectively apply the brakes and stop the vehicle without collision with the obstruction on the road.
Stopping Sight Distance = Lag Distance + Braking Distance = v.t + v^2/2gf where, v is in m/sec
(b) Overtaking Sight Distance: Overtaking Sight Distance is the sight distance which is required by the vehicle running at the design speed to overtake a vehicle running in the same lane at relatively smaller speed.
OSD = d1+ d2+ d3 = vb.t + vb.T + 2S + v.T
(c) Sight Distance at the Intersection: Sight distance on the uncontrolled intersections should be sufficient enough for the safety of the traffic, it should be at least equal to the stopping sight distance.
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