Overtaking Sight Distance with Concept and Formula

Overtaking sight distance is the sight distance required by a fast-moving vehicle to safely overtake a slow-moving vehicle. It is also called the safe passing sight distance. The detailed concept and formula for calculating the overtaking sight distance are covered in this blog.

Need for Overtaking Sight Distance

In mixed traffic flow conditions, not all vehicles will move at the design speed. Therefore, it becomes necessary for fast-moving vehicles to overtake the slow-moving vehicle. Also, it may not be possible to provide overtaking opportunities throughout the stretch of the road, but it must be provided at regular distance intervals.

By considering the statements above, overtaking sight distance can be reframed as, "The minimum sight distance open to the vision of the driver of a vehicle intending to overtake a slow vehicle ahead with safety against the traffic of opposite direction."

Factors Affecting Overtaking Sight Distance

• Speeds of overtaking vehicle, overtaken vehicle, and the vehicle coming from the opposite direction

• Distance between the overtaking and overtaken vehicles

• Rate of acceleration of overtaking vehicle

• The gradient of the road

• Skill and reaction time of the driver

Overtaking Manoeuvre

The overtaking manoeuvre is the step that typically happens in an overtaking operation. Below given is the pictorial representation of the overtaking operation.

From the diagram it is clear that the red box represents an overtaking vehicle, the yellow box is the overtaken vehicle, and the green represents the vehicle in the opposite direction.

The distances d1, d2, and d3 are explained as follows.

• d1 - distance travelled by overtaking vehicle(A) during the reaction time 't' of the driver from position a1 to a2

• d2 - distance travelled by the overtaking vehicle from a2 to a3 during the actual overtaking operation in time 'T'

• d3 - distance travelled by the vehicle in the opposite direction(C) from c1 to c2 during the overtaking operation in the same time taken in d2 i.e. 'T'

Assumptions in Calculating Overtaking Sight Distance

1. Vehicle A is forced to reduce the speed to that of the vehicle being overtaken (vb) and moves behind it allowing space 's' till there is an opportunity to overtake.

Therefore, d1 = vb * t

2. From position a2, vehicle A accelerates, moves to the adjacent lane, overtakes and joins the lane at a distance 's' from the overtaken vehicle in time 'T'

d2 = 2*s + b, where, s - minimum spacing between the vehicles

s = (0.7*vb + 6), in m

b = vb*T, in m

The time 'T' depends on the speed of the overtaken vehicle and the acceleration of the overtaking vehicle. This can be found by equating d2 to equations of motion, vbT + 0.5*a*T^2, where a is the acceleration of vehicle 'A' in m/sec^2

d2 = b + 2s = vbT + 0.5*a*T^2, where, b = vb*T,

Therefore, 2s = 0.5*a*T^2

T = (4s/a)^0.5

3. Distance moved from c1 to c3 is the distance moved by vehicle 'C' at design speed 'v m/s' in time 'T'

d3 = v*T, in m

Thus the overtaking sight distance is given as,

OSD = d1 + d2 + d3 = vb*t + vb*T + 2*s + v*T

If velocities are given in kmph units,

OSD = 0.28*vb*t + 0.28*vb*T + 2*s + 0.28*v*T

where,

• vb - speed of the overtaken vehicle, in kmph = (Design speed (V) - 16) in kmph, if not given in the question

• V - design speed, in kmph

• Reaction time, t = 2 seconds

• T = (14.4*s/A), in seconds

• A - acceleration, in kmph

Overtaking Zones

Having OSD in every section of highways is highly impossible. In such cases, overtaking zones can be provided at regular intervals for overtaking operations to take place. The OSD and sufficient pavement width should be there in these zones.

The length of OSD zones should be a minimum of 3 times the minimum overtaking sight distance (OSD). But it is desirable to have a length of 5 times the minimum overtaking sight distance.

It is necessary to provide signposts to indicate the beginning and end of overtaking zones. Signposts must be placed at least OSD distance before the start and end of the overtaking zones.

Note:

• For one-way traffic roads, there is no vehicle coming in the opposite direction. Therefore, OSD is simply d1+d2 only.

• OSD is not required for four or more lane roads with divided zones.

Practise Problem

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