• Sight Distance at Intersection

Uncontrolled Intersection

On all approaches to interesting roads,there should be a clear view across the corners from a sufficient distance, so as to avoid collision.

The design sight condition

1. Enabling the approaching vehicle to change speed (slow down).

2. Enabling the approaching vehicle to stop.

3. Enabling stopped vehicles to cross the road.

4. The sight distance should be sufficient to enable either one or both approaching vehicles to change the speed to avoid collision.

5. The vehicle approaching from the minor road must slow down

6.  The total reaction time required for the driver to decide to change speed is 2 sec (assumed) & at least 1 sec more is needed for making the change in speed.

7. Hence in this ease sight distance  would be equal to distance traveled by vehicles at design speed in 3 sec.

The distance for the approaching vehicle should be sufficient to bring either both or one of the vehicles to a stop before reaching the point of conflict.

Hence it would be equal SSD

In this case vehicles entering the intersection from  the minor road are controlled by stop or give away signs so the vehicle has to stop & then proceed to cross the main road.

In this case the total time required by a stopped vehicle would include to start, accelerate and cross the main road before another vehicle traveling at a design speed on main road reaches the intersection.

For safety consideration the sight distance at uncontrolled section is max of above three cases.

• Controlled Intersection

• It is signalized intersection the above three requirements are not applicable

• At rotary intersection the sight distance should be at least equal to the SSD for design speed of rotary.