It is the minimum distance open to the view of the driver of a vehicle intending to overtake a slow vehicle ahead with safety against the traffic in the opposite direction. The overtaking sight distance is measured along the centre line of the road which a driver with his eye level 1.2m above the road surface.
Important factors on which the minimum overtaking sight distance or the safe passing sight distance depends are:
(i) Speeds of the overtaking , overtaken and the vehicle coming from the opposite direction.
(ii) Spacing between the vehicles.
(iii) Skill and reaction time of the driver , and the (iv) Slope of the road.
3.7.1. Analysis of Overtaking Sight Distance:
Figure 3.7 shows the elements that go to make up the overtaking sight distance. In Fig 3.7 A is the ‘overtaking vehicle’ travelling at the design speed and B is the ‘overtaken vehicle’ moving slowly on a two lane road. The vehicle C is the ‘on coming vehicle’ coming in the opposite direction at design speed.
Fig 3.7 Elements of taking manoeuvre The overtaking distance may be divided into three parts:
(i) d1 - the distance travelled by the overtaking vehicle A. during the reaction time ‘t’ from position A1to A2.
(ii) d2 - the distance travelled by vehicle , A from A2 to A3 during the actual overtaking operation.
(iii) d3 is the distance travelled by the on-coming vehicle C , from C1 to Cz during the overtaking operation of A.
The vehicles A and C are travelling at a design speed of V kmph or v m / sec.
The overtaken vehicle B is moving slowly at a speed of Vbkmph or vbm / sec.
In a two-way lane road , the opportunity to overtake depends on the frequency of vehicles coming from the opposite direction and overtaking sight distance available at any instant.
The overtaking phenomenon may be assumed as follows:
(i) When it is decided to overtake , the overtaking vehicle A , reduces its speed to the speed of the slow moving vehicle B and moves behind it during the reaction time t , till there is opportunity for safe overtaking.
This distance d1, shown in fig 3.7 = vbx t (metres) where t is the reaction time of the driver in seconds.
This reaction time is taken as 2 seconds as an average value.
Then d1= vbx t = vbx2 metres 3.6
(ii) From position A2 , the vehicle A starts accelerating , shifts to the adjoining lane , overtakes the vehicle B , and shifts back to its original lane ahead of B in position A3 . The distance between the positions A2and A3is taken as d2.
From the geometry of the figure d2 = b + 2s where s is the spacing to be maintained between the vehicles and is given by the formula.
s = (0.7 vb+ 6) metres 3.7
Let the time taken by vehicle A to overtake vehicle B be T sec. During this time the vehicle A moves from A2to A3over a distance of d2and the vehicle B , moves from B1to B2
over a distance b.
Then b = vb. T (metres) since vbis the speed of vehicle B.
Thus d2= vbT + 2s (from the geometry , fig 3.7) 3.8.
The vehicle A , travelling with an initial velocity of vb, accelerated at ‘a’ metres / sec , travels a distance of d2in time T seconds , then
d2 = vbT + 1 / 2 (aT2) 3.9
Equating equations 3.8 and 3.9.
d2= vbT + 2S = vbT + 1 2 aT2
or Tsec =
4s / a
3.10where s = (0.7 vb+ b) from equation 3.7.
Hence d2= vbT + 2s 3.10.
(iii) During the reaction Time T seconds , the on coming vehicle travelling with a speed of v m / sec covers a distance d3given by
d3= v T metres 3.11
The overtaking distance is the sum of d1, d2and d3and hence O.S.D. (metres) = d1+ d2+ d3
= vbt + (vbT + 2s) + vT
= Vbx2+ (vbT + 2s) + vT 3.12a = 2 x 0.278 VB+ 0.278 VbT + 2S + 0.278 VT 3.12b where Tsec =
14.4S / A
S = (0.2 Vb+ 6) metres - spacing between the vehicles A = Acceleration in kmph / sec.
Table 3.9 may be used for finding max. acceleration of vehicles at different speeds.
Table 3.9 Maximum Overtaking Accelerations:
Speed Max. Overtaking Acceleration V Kmph v m / sec. A Kmph / sec. a m / sec2
25 6.93 5.00 1.41
30 8.34 4.80 1.30
40 11.10 4.45 1.24
50 13.80 4.00 1.11
60 18.00 3.28 0.92
80 22.20 2.56 0.72
100 27.80 1.92 0.53
In case the speed of the overtaken vehicle is not given , Vb may be assumed as (V - 16) kmph or vb= (v - 4.5) m / sec.
At overtaking sections the minimum overtaking sight distance should be (d1+ d2+ d3) when two way traffic exists. On divided highways , the overtaking sight distance need be only (d1+ d2) as in one way movements as no vehicle is expected to come from the opposite direction.
The IRC suggests that , on divided highways with four or more lanes , it is not necessary to provide usual overtaking sight distance ; however the sight distance in such highways should be more than the stopping sight distance.
3.7.2. Effect of Grades on Overtaking Sight Distance:
Appreciable grades in the road , both ascending as well as descending , increase the sight distance required for safe overtaking. On up grades , the acceleration of the overtaking vehicle will be less and hence the passing will be difficult , but the overtaken vehicle may also decelerate in steep ascending gradients and compensate to some extent the passing sight distance requirement.
In down grades though it is easier for the overtaking vehicles to accelerate and pass ; the overtaken vehicle may also accelerate and cover greater distance ‘b’ during the overtaking time.
Therefore , the overtaking sight distance , at both ascending and descending grades are taken as equal to that at level stretch. However , at grades , it is desirable to provide overtaking sight distance more than that required at level.
3.7.3. Overtaking Zones:
It is desirable to construct highways such that the length of the road visible ahead at every point is sufficient for safe overtaking. This is seldom practicable and there may be stretches where the safe over taking distance cannot be provided. In such zones where overtaking is not possible , sign posts should be installed indicating “Overtaking Prohibited”
before such restricted zones start. But the overtaking opportunity for vehicles moving at design speed should be given at frequent intervals. These zones which are meant for overtaking are called “Overtaking Zones”.
Fig 3.8 shows an overtaking zone. The minimum length of the zone is to be 3 times the overtaking sight distance. It is desirable to keep the length of the zone as five times the overtaking sight distance. At these sections , there should be sufficient pavement width for safe overtaking operations. The beginning and end of these zones should be indicated well ahead by installing suitable sign boards.
Fig 3.8 Overtaking Zone WORKED EXAMPLE:
3.3. Calculate the passing sight distance for a two way traffic high way for which the design