It is the actual distance along the road at which a driver has visibility of stationary or moving object from a specified height above the carriage way. In other words it is the length of road visibility ahead to the driver at any instant. The standard height depends upon the line of sight of the driver above the road surface, when the visibility is measuring on level surface,horizontal curve or on the inclined of the vertical curve. The standard height of a line of sight of the driver above the road surface is taken as 1.2m. The SD depends upon the following.
1. Stopping Sight Distance (SSD)
The min SD required by a vehicle travelling at a given speed to bring his bring his vehicle to stop after an object on carriage way because becomes visible is called SSD. The minimum SSD is equal to the sum of reaction distance and breaking distance.
2. Overtaking Sight Distance (OSD)
The minimum distance required for overtaking another vehicle safely without interfering the speed of an opposing vehicle travelling at the design speed is called overtaking or safe passing SD.
40 km/h 165 meters
100 km/h 640 meters