Overtaking sight distance

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.

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1. OVERTAKING SIGHT DISTANCE PRESENTED BY AGLAIA

2. OVERTAKING REQUIRMENT All vehicles dont travel at design speed  Need to overtake slow moving traffic by fast moving vehicles Adequate sight distance for overtaking should be provided along the alignment The minimum distance open to the vision of the driver of a vehicle intending to overtake slow moving vehicle ahead with safety against the traffic in opposite direction is called overtaking sight distance

3. OVERTAKING REQUIRMENT The overtaking sight distance (OSD) is the distance measured along the centre of the road which a driver with his eye level at 1.2 m above the road surface can see an object 1.2 m above the road surface

4. FACTORS AFFECTING OSD 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

5. ANALYSIS OF OSD (2 WAY ROAD) A vehicle (A) travelling at design speed v  desires to overtake a slow moving vehicle B travelling at vb The vehicle A has to wait till he get an opportunity to overtake B  accelerate , shift to the opposite lane and complete the overtaking Return to the left lane without colliding with opposite coming vehicle C Fig is shown in next slide

6. ANALYSIS OF OSD (2 WAY ROAD)

7. Overtaking may be split into 3 operations  d1 is the distance travelled by the overtaking vehicle A during the reaction time t of the driver from position A1 to A2 just before starting to overtake B d2 is the distance travelled by the overtaking vehicle A during the actual overtaking operation during time T d3 is the distance travelled by the vehicle coming in the opposite direction (C) during the actual overtaking operation

8. Overtaking operation

9. Assumptions in analysis The overtaking vehicle is forced to reduce its speed from design speed to the speed of overtaken vehicle (B ) & move behind it at a spacing s till an opportunity for safe overtaking operation When driver of vehicle A finds a gap  decides within his reaction time (t) to accelerate and overtake B  during this time vehicle A moves with a speed of vb from A1 to A2 (d1) The vehicle A accelerate and overtake B within a distance d2  during time interval T

10. Assumptions in analysis Distance d2 is split into 3 parts Spacing s between A2 and B1 Distance b travelled by slow moving vehicle (B1 to B2) Spacing s between B2 and A3 During overtaking time T, vehicle C in opposite lane travel from C1 to C2 through a distance of d3

11. Determination of components d1 = t * vb vb – velocity of slow moving vehicle d1 = 2 vb t – reaction time ( t = 2 sec) d2= s + b + s s – spacing between vehicles (m) d2 = 2s + b s = 0.7 vb + 6 metre b = vb * T (T overtaking time) (T= sqr {(4*s)/a} ) a – rate of acceleration d2 = vb * T + 2 (0.7 vb + 6) d3= vT T – overtaking time (sec) v- design speed If vb not given vb = v – 4.5 m/sec

12. OSD = d1 + d2 + d3

13. Rate of acceleration Vs speed

14. Overtaking zones It is not possible to construct the road with OSD at every point There may be stretches where overtaking is not possible However overtaking option should be given to vehicle moving at design speed at frequent intervals as possible These zones are called overtaking zones The width of carriage way will be more at those zones Width and length should be sufficient for such zones for safe overtaking DESIRABLE LENGTH OF OVERTAKING ZONE = 3 OSD MINIMUM LENGTH OF OVERTAKING ZONE = 5 OSD

15. Overtaking zone Sign board @ OSD distance at beginning and end of zone

16. Overtaking zone NON OVERTAKING

17. Intermediate sight distance If OSD cannot be provided  provide ISD ISD= 2 OSD Head light sight distance The distance visible to a driver during night driving due to head light HSD = SSD

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