Highway Geometric Design Overtaking Sight Distance (OSD)

Definition: The minimum distance open to the vision of a driver of a vehicle intending to overtake slow vehicle ahead with safety against the traffic of opposite direction is known as

 ‘Minimum Overtaking Sight Distance’ or ‘Safe Passing Sight Distance’.

As per IRC : The distance measured along the centre line of road which a driver with his eye level at 1.2 m above road surface can see the top of an object 1.2 m above road surface.

Formulation of OSD:

d1 = distance traveled by overtaking vehicle ‘A’ during reaction time ‘t’ from position ‘A1’to ‘A2’.

d2 = distance traveled by vehicle ‘A’ from ‘A2’ to ‘A3’ during actual overtaking operation.

d3 = distance traveled by oncoming vehicle ‘C’ from ‘C1’ to ‘C2’ during the overtaking operation of ‘A’.

• d1 = Vb .t (As vehicle A reduces speed to that of B)                                                                         t = reaction time of driver in OSD=2 sec                                                                                       Vb = speed of slow vehicle in m/sec.

Note: If ‘Vb’ is not given, it may be assumed as

Vb = (V — 16)  kmph or (V- 4.5) m/s

Important Note:

• For two way traffic, OSD=d1 +d2+d3.
• For one way traffic or divided highway OSD = d1 + d2.

Minimum length of overtaking zone:

• =3 (O.S.D)
• = 3 (d1 + d2), for one way roads.
• = 3 (di + d2 + d3), for two way roads.

Note: It is desirable that the length of overtaking zone is five times the O.S.D

Overtaking accelerations as per IRC: