Sight distances | SSD and OSD

 SIGHT DISTANCES | SSD and OSD
 It is defined as the length of the road visible to the driver a head of him at any time instant. It can be divided into various types  such as : -

Stopping Sight Distance

 The minimum sight distance available on highway at any spot so that the driver is able to stop his vehicle when he sees any obstruction without causing any accident or collision. As per IRC guidelines, the height of the eye level of the driver above the payment surface must be 1.2 metre and height of the object must be 0.15 metre.
 The stopping sight distance depend upon the following factors like reaction time, speed of the vehicle, efficiency of brakes and frictional resistance. The reaction time depends upon PIEV theory where P stand for perception time,  I stand for intellection time,  E stand for emotion time and V stand for violation time.
 The speeds is also is an important factor for deciding SSD. More
is the speed more will be the SSD.
 Braking efficiency is also important and idealy. It should be 100%.
 Therefore, the total stopping sight distance is : -

SSD = vt + {v^2/ 2g (f +n )}

 v =  design speed in metre per second ( m/s) 

 t= reaction time (2.5 sec)  

 f = design coefficient of friction (0.35 0.4) 

 n = slope or gradient 

Overtaking Sight Distance 

It is the minimum distance provided in the highway design so that the driver of the vehicle moving with the design speed which is able to overtake a slow moving vehicle ahead without possibilities of accidents with another vehicle coming from opposite direction.

 As per IRC norms the height of the high level of the driver is considered to be 1.2 m above the road level and height of the top of the object  is also same i.e., 1.2 m.

OSD =  v't + v'T +2S + v.T