OVERTAKING SIGHT DISTANCE (OSD) the minimum distance required for overtaking operation with safety. OSD is also called safe passing distance available As per IRC the distance measured along the centerline of the road which a driver with his eye level at 1.2m above road surface can see the top of an object 1.2m above the road surface.

Overtaking Sight Distance (OSD) Depends Upon:

  • Speed of overtaking, overtaken and a vehicle coming from the opposite direction
  • The spacing between overtaking and overtaken vehicle
  • Skill and reaction time of the driver
  • Rate of acceleration of overtaking vehicle
  • The gradient of the road if any


 d_{1} \: = distance\: travelled\: by\: overtaking\: by\: vehicle\: A, \:during\: the\:reaction \:time \:of\: the\: driver \:from\: position \:A_{1} \:to \:A^{2}

 d_{2}= distance\: travelled\: by \:the \:vehicle\: A\: from\: A_{2}\: to \:A_{3}\: during \:the\: actual\: overtaking\: operation\: in\: time\: T\: seconds.

 d_{3}= distance\: travelled\: by\: coming\: vehicle \:C\: from\: C_{1}\: to\: C_{2}\: during \:the \:overtaking \:operation\: of \:A\: i.e \:T \:seconds.

 Assumptions\: made\: during\: calculation\: of\: d_{1},\: d_{2} \:and \:d_{3}\: are:

(1) It is assumed that the vehicle A is forced to reduce. Its speed to the speed vb of the slow vehicle B and behind it allowing a Space S, till there is an opportunity for Safe overtaking operation. The distance travelled by vehicle A during this reaction time is d1. This distance will be vb .t where T is the reaction time ≈ 2sec

(2) From position A2 vehicle A starts accelerating, shifts to adjoining lane, Overtakes the vehicle B, and shifts back to Its original lane head of B in position A3 in Time T sec. This is distance d2 (between A2 to A3).

  • The minimum distance between A2 and B1= distance between A3 and B2, which may be taken as minimum spacing S of the two vehicles while moving with vb (m/sec)
  •  S = (0.7v_{b}+6)……………..(v_{b}=m/sec)

If the time taken by vehicle A for the overtaken operation A2 to A3 is T sec. The distance covered by B in this time is

 b = v_b.T

 d_{2} = b \:+ \: 2s \:+ \: = v_{b}.T \: + \: \frac{1}{2}.a.T^{2} .................................. (1)

Where a is acceleration in m/sec2

From equation (i)

 2S = \frac{1}{2}.a.T^{2} \: \: \: . \: \: \: \: T = \sqrt{\frac{4S}{a}}

 Hence \: d_{2} = T.v_{b} + 2 S

(3) The distance travelled by vehicle C moving at Design speed v m/sec during the overtaking Operation of A.

 d_{2} = v.t

  ODS = d_{1} \: + \: d_{2} \: + \: d_{3}

 ODS = v_{b}.t \: + \: (T.v_{b} \:+\:2S)+ \: v.T

 Where v_{b} is in m/sec

 OSD =0.278 V_{b}.t + (0.278 V_{b}.T +2S) + 0.278V.T

Where Vb is kmph.

 T = \sqrt{\frac{4S}{a}} where \: a \: as \: in \: m/sec^{2}

NOTE: If vb is not given
Vb = V-16 (kmph)
Vb = V-4.5 (m/sec) can be taken
Minimum length of Overtaking Zone=3 Overtaking Sight Distance (OSD)
Desirable overtaking Zone =5 Overtaking Sight Distance (OSD)

During overtaking operation, automobile C actions at layout pace v cowl the distance= d3= v×T.Total overtaking distance=d1+d2+d3.

If the street is unmarried facet site visitors lane then there may be no automobile C that comes in the contrary direction. Thus d3 is neglected. Then overall overtaking sight distance= d1+d2.

Safe Overtaking sight distance = 5* overall overtaking distance calculated whilst there may be no limit of land.   (OR)                                                 = 3* overall overtaking sight distance whilst there may be a limit of land.

Overtaking Sight distance is the minimal distance required with the aid of using car shifting at layout velocity to overhaul the car shifting at slower velocity correctly in opposition to every other car coming from a contrary direction. It relies upon the following factors:

  • Speed of overtaking a car ( v m/s)
  • Speed of overtaken car ( vb m/s)
  • Speed of contrary car ( v m/s)
  • The reaction time of overtaking car driver
  • Spacing among the vehicles (s)
  • Acceleration of overtaking a car (m/s2)
  • The gradient of the road


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