SIGHT DISTANCE 

The safe and efficient operation of vehicles on the road depends very much on the visibility of the road ahead of the driver.

Thus the geometric design of the road should be done such that any obstruction on the road length could be visible to the diver from some distance ahead. This distance is said to be the sight distance. In other words the feasibility to see ahead, or the visibility is very important for safe vehicle operation on a highway.

TYPES OF SIGHT DISTANCE 

Sight distance is available from a point is the actual distance the road surface, over which a driver from a specified height above the carriage way has visibility or moving objects.

The sight distance situations that are considered for design are :

1. Stopping sightdistance (SSD) or the absolute minimum sight distance
2. Intermediate sight distance (ISD) is defined as twice SSD
3. Overtaking sightdistance (OSD) for safe overtaking operation
4. Head light sight distance is the distance visible to a driver during night driving under the illumination of head lights.
5. Safe sight distance to enter into an intersection.

(a) Sight distance at Horizontal Curve 

(b) Sight distance at Vertical Summit Curve

(c) Sight distance at Intersection

The standards for sight distance should satisfy the following three conditions :

                              i.            Driver travelling at the design speed has sufficient sight distance or length of road visible ahead to stop the vehicle, in case of any obstruction on the road ahead, without collision.

                           ii.            Driver travelling at the design speed should be able to safely overtake, at reasonable intervals, the slower vehicles without causing obstruction or hazard to traffic of opposite direction.

                        iii.            Driver entering an uncontrolled intersection (particularly unsignalised  intersection) has sufficient visibility to enable him to take control of his vehicle and avoid collision with another vehicle

The computation of sight distance depends upon :

Reaction time of driver : 

Reaction time of driver is the time taken from the instant the object is visible to the driver to the instant when the brakes are applied. The total reaction time may be split up into four components based on PIEV Theory. IRV suggests a total reaction time of 2.5 sec. 2.5 sec is actually the 90^th percentile reaction time.

Speed of vehicle : 

Higher the speed, more time will be required to stop the vehicle. Hence it is evident that, as the speed increases, sight distance also increases.

Efficiency of Brakes :

The efficiency of brakes depends upon the age of the vehicle, vehicle characteristics etc. if the brake efficiency is 100%, the vehicle will stop the moment the brakes are applied. But practically, it is not possible to achieve 100% brake efficiency. Therefore, the sight distance required will be more, when the efficiency of brakes is less. Also for safe geometric design, we assume that the vehicles have only 50% brake efficiency.

Gradient : 

Gradient of the road while climbing up a gradient, the vehicle can stop immediately. Therefore, sight distance required is less. While descending a gradient, gravity also comes into action and more time will be required will more in case.