What is Sight Distance
The distance along the center line of the road at which a driver has visibility of an object, stationary or moving at a specified height above the carriage way is known as sight distance.
Sight distance is the length of road visible ahead to the driver at any instance. The standards for sight distance should satisfy the following conditions :
- Driver travelling at the design speed has sufficient sight distance or road length visible to stop vehicle, in case of any obstruction on the road ahead, without collision.
- Driver travelling ahead should be able to overtake slow moving vehicles, at reasonable intervals, without causing obstruction or hazard to traffic of opposite direction.
- The driver entering a uncontrolled intersection, he should have sufficient visibility to enable him to control his vehicle in order to avoid collision with another vehicle.
Factors Affecting Sight Distance
The sight distance depends upon following factors
- Speed of the vehicle.
- The frictional resistance of the road surface.
- Height of drivers eye.
- Slope of the road surface.
- Perception time and brake reaction time.
Types of Sight Distance
Following `types of sight distances are considered for geometrical design of road :
- Stopping or non-passing sight distance
- Overtaking or passing sight distance
- Intermediate sight distance
- Lateral sight distance.
1. Stopping or Non-passing Sight Distance :
The clear distance ahead needed by a driver to bring his vehicle to a stop before meeting a stationary object on the road is called as stopping or non-passing sight distance.
Below are given some of the examples of sight distance at various situations of road.
1. Sight Distance @ Horizontal Curve
2. Sight Distance @ Road Intersection
3. Sight Distance @ Vertical Summit Curve
In this case of a summit curve, minimum stopping sight distance is the distance measured along the center line of a road at which a drive whose eye sight is 1.22 m above the road surface can see the top of an object 15 cm high on the road.
Below table shows the various safe stopping sight distances for various design speeds :
| Speed | Perception and Brake Reaction | Braking | Safe Stopping Sight Distance | |||
|---|---|---|---|---|---|---|
| V (km/h) | Time t (sec) | Distance (m) d1=0.278 Vt | Coefficient of longitudional friction (f) | Distance (m) d2= V²/254 f | Calculated values (d1 + d2) | Design Values (m) |
| 20 | 2.5 | 14 | 0.40 | 4 | 18 | 20 |
| 25 | 2.5 | 18 | 0.40 | 6 | 24 | 25 |
| 30 | 2.5 | 21 | 0.40 | 9 | 30 | 30 |
| 40 | 2.5 | 28 | 0.38 | 17 | 45 | 45 |
| 50 | 2.5 | 35 | 0.37 | 27 | 62 | 60 |
| 60 | 2.5 | 42 | 0.36 | 39 | 81 | 80 |
| 65 | 2.5 | 45 | 0.36 | 46 | 91 | 90 |
| 80 | 2.5 | 56 | 0.35 | 72 | 128 | 130 |
| 100 | 2.5 | 70 | 0.35 | 112 | 182 | 180 |
2. Overtaking Sight Distance (OSD) :
The minimum distance open to the vision of the driver on a two way road to enable him to overtake another vehicle ahead with safety against the traffic from opposite direction is called overtaking or passing sight distance.
The minimum over taking sight distance depends upon the following factors:
- Speed of overtaking, overtaken and the speed of the vehicle coming from the opposite direction.
- Distance between the vehicles.
- Rate of acceleration of the overtaking vehicle.
- Skill and reaction time of the driver.
Theoretical Derivation of OSD
- Consider a two lane straight road with three vehicles A, B and C as shown in Fig.
- Let the speed of fast moving vehicle A be V1 m/sec and that of slow moving vehicle B be V2 m/sec. Let a third vehicle C be coming from opposite direction with a speed of V3 m/sec.
- Assume V = design speed in m/sec.
∴ V = V1 ⊂ V3
- In order to overtake vehicle B, driver of vehicle
‘A’ should apply the following criteria :
- He should slow down his speed to V2 and come out on second lane. When he begins to come out, vehicle ‘A’ should be at a safe distance ‘S’ behind vehicle ‘B’.
- Driver at ‘A’ should accelerate his vehicle on second lane ‘A’ till vehicle ‘B’ is overtaken. He should then run his vehicle back on his original first lane, ahead of vehicle ‘B’ and at this instant, vehicle ‘A’ should be again at a safe distance and a head of vehicle ‘B’.
- In this process of overtaking he must avoid collision with vehicle ‘C’ because then he is moving with the acceleration on the second lane on which vehicle ‘C’ is coming from the opposite direction.
Then, L = Length of OSD
L = d1 + d2 + d3
Where,
d1 = The distance travelled by the overtaking vehicle ‘A’ on the first lane before it comes out on second lane for the purpose of overtaking during this it slows down.
Its speed V2, by allowing space ‘S’ is a safe spacing between A and B,
∴ d1 = (V2 × t)
d2 = The distance by vehicle ‘A’ during the total overtaking time while coming from first lane to second lane, accelerating along the second lane and coming back to,
First lane = (2S × V2 × t0)
Where,
t0 = Time taken in overtaking, with acceleration ‘a’ m/sec² to cover a distance of (d2 – 2S).
d3 = Distance travelled by vehicle ‘C’ on second lane during the accelerating time, t0 = (V x t0).
- Substituting for d1, d2, and d3 in Equation, L = (V2 x t) + (2S + V2 x t0) + (V x t0) = [ V2t +(Vst0+2S) + V t0]
- The safe spacing ‘S’ to be kept between vehicle ‘A’ (fast moving) and vehicle B (slow moving) is given by expression. S = ( V2 × 0.7 ) + length of vehicle ‘A’ (assume 6m) = 0.7 V2 + 6.
- The value of t0 is obtained by the following equation, t0 = 2√S/a, here a = acceleration in m/sec².
Below table shows the overtaking sight distance for various speed as recommended by IRC
| Sr. no. | Speed in Km/h | Time Component in Seconds | |||
| For Overtaking Manocurve | For Opposite Vehicle | Total | Safe OSD in meters | ||
| 1 | 40 | 9 | 6 | 15 | 165 |
| 2 | 50 | 10 | 7 | 17 | 235 |
| 3 | 60 | 10.8 | 7.2 | 18 | 300 |
| 4 | 65 | 11.5 | 7.5 | 19 | 340 |
| 5 | 80 | 12.5 | 8.5 | 21 | 470 |
| 6 | 100 | 14 | 9 | 23 | 640 |
3. Intermediate Sight Distance (ISD) :
The distance which affords reasonable opportunities to drivers to overtake the vehicle ahead with caution is known as intermediate sight distance. ISD is taken as twice the safe stopping distance.
Design Values of ISD as recommended by IRC are given below
| Speed (Km/h) | Intermediate Sight Distance | Speed (Km/h) | ISD (m) |
|---|---|---|---|
| 20 | 40 | 50 | 120 |
| 25 | 50 | 60 | 160 |
| 30 | 60 | 65 | 180 |
| 35 | 80 | 80 | 240 |
| 40 | 90 | 100 | 360 |
4. Lateral Sight Distance :
The sight distance needed by the driver of a vehicle who see another vehicles approaching the intersection, reacts and applies brakes to bring his vehicle to dead stop at the intersection without any collision or accident is called safe sight distance for entering into an intersection or lateral sight distance.
The lateral sight distance should be sufficient to satisfy the following three conditions :
- To enable both or at least one approaching vehicles to change their speeds to avoid collision.
- To bring at least one or both the approaching vehicles to stop before reaching a point of
collision. - To enable the stopped vehicles on minor road to start, accelerate and cross the main road before the approaching vehicle, traveling at design speed on main road, reaches the interaction.
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