Overtaking Sight Distance (OSD)

The overtaking sight distance is the minimum distance open to the vision of the driver of a vehicle intending to overtake the slow vehicle ahead safely against the traffic in the opposite direction. The overtaking sight distance or passing sight distance is measured along the center line of the road over which a driver with his eye level 1.2 m above the road surface can see the top of an object 1.2 m above the road surface.

The factors that affect the OSD are:

• Velocities of the overtaking vehicle, overtaken vehicle and of the vehicle coming in the opposite direction.
• Spacing between vehicles, which in-turn depends on the speed
• Skill and reaction time of the driver
• Rate of acceleration of overtaking vehicle
• Gradient of the road

The dynamics of the overtaking operation is given in the figure which is a time-space diagram. The x-axis denotes the time and y-axis shows the distance traveled by the vehicles. The trajectory of the slow moving vehicle (B) is shown as a straight line which indicates that it is traveling at a constant speed. A fast moving vehicle (A) is traveling behind the vehicle B. The trajectory of the vehicle is shown initially with a steeper slope.

The dotted line indicates the path of the vehicle A if B was absent. The vehicle A slows down to follow the vehicle B as shown in the figure with same slope from t0 to t1. Then it overtakes the vehicle B and occupies the left lane at time t3. The time duration T = t3 - t1 is the actual duration of the overtaking operation. The snapshots of the road at time t0; t1, and t3 are shown on the left side of the figure. 

Time space Diagram

From the Figure given above, the overtaking sight distance consists of three parts.

d1 the distance traveled by overtaking vehicle A during the reaction time t = t1 - t0

d2 the distance traveled by the vehicle during the actual overtaking operation T = t3 - t1

d3 is the distance traveled by on-coming vehicle C during the overtaking operation (T).

Therefore:

OSD = d1 + d2 + d3

It is assumed that the vehicle A is forced to reduce its speed to vb, the speed of the slow moving vehicle B and
travels behind it during the reaction time t of the driver. So d1 is given by:

                 d1 = vb t

Then the vehicle A starts to accelerate, shifts the lane, overtake and shift back to the original lane. The vehicle

A maintains the spacing s before and after overtaking. The spacing s in m is given by:

s = 0.7vb + 6

Let T be the duration of actual overtaking. The distance traveled by B during the overtaking operation is 2s+vbT. 

Also, during this time, vehicle A accelerated from initial velocity vb and overtaking is completed while reaching final velocity v. Hence the distance traveled is given by:

The distance traveled by the vehicle C moving at design speed v m=sec during overtaking operation is given by:

d3 = vT

The the overtaking sight distance is 

where vb is the velocity of the slow moving vehicle in m=sec2, t the reaction time of the driver in sec, s is the spacing between the two vehicle in m given by equation

s = 0.7vb + 6

and a is the overtaking vehicles acceleration in m/sec2. In case the speed of the overtaken vehicle is not given, it can be assumed that it moves 16 kmph slower the design speed.

The acceleration values of the fast vehicle depends on its speed and given in Table. 

Note that:

• On divided highways, d3 need not be considered
• On divided highways with four or more lanes, IRC suggests that it is not necessary to provide the OSD, but only SSD is sufficient.

Overtaking zones

Overtaking zones are provided when OSD cannot be provided throughout the length of the highway. These are zones dedicated for overtaking operation, marked with wide roads. The desirable length of overtaking zones is 5 time OSD and the minimum is three times OSD.