IRC:66 RECOMMENDS ABSOLUTE MINIMUM SIGHT DISTANCE IS THE SAFE STOPPING SIGHT DISTANCE, BASED ON THE DESIGN SPEED, KM /HOUR.

Geometrical design standards generally represent an acceptable compromise between the road users requirements and the construction costs, where in stopping distance is the distance covered by a moving vehicle from the instant an obstacle on the road ahead becomes visible to the driver and the vehicle is brought to a stop. Minimum safe stopping distance is known as non -passing sight distance. Where in Visibility distance is the length of a road ahead of the vehicle which should be visible to the driver to enable him to bring his vehicle to a stop in case of an obstruction on the road (A driver can see at a distance of about 300 metres in line with his eye along a level road) . The minimum sight distance should be based on expected speed of the vehicles and the breaking distance required to stop and which should also include the drivers reaction time to the danger. The safe sight distance is dependent on the breaking time of the vehicle and is generally taken as being equal to twice the distance required for a vehicle to stop. Mechanical and human factors should be taken into account-- the breaks may be defective, the driver may be drunk and these factors will need more of stopping distance and reaction time. The safe stopping sight distance should invariably be provided throughout the length of all roads. The sighting distance provided on roads should not be less than the safe Stopping distance for the particular speed allowed on the road. Where the minimum stopping distance cannot be provided, warning boards should be fixed for reducing the speed. Absolute minimum sight distance is the safe stopping sight distance and as per the guidelines based on IRC :66 , In single lane roads where two way movement of traffic is permitted the minimum stopping sight distance should be equal to twice the stopping distance to enable both vehicles coming from opposite directions to see each other. Restrictions to sight distance may be caused at horizontal curves or in vertical summit curves, by objects obstructing vision at the inner side of the road or at intersections. As far as overtaking sight distance is concerned, the distance required for a moving vehicle to overtake and safely pass another vehicle moving in the same direction but at a lower speed. The minimum distance ahead that must be clear to permit safe passing is called the passing sight distance. Where the step gradients to the approaches a vertical curve can not be avoided, the summit of the curve (hump) should be made horizontal for a length of about 30 metres. Same visibility is recommended on horizontal curves as on vertical curves. The sight distance is measured along the centre line of the road. Roads of 3.75 metres width and less where there are no berms such as canal and the village roads, overtaking places of not less than 61 metres should be provided at frequent intervals. Visibility across the inside of horizontal curves is very important, lack of visibility in the lateral direction may arise due to obstructions like walls, cut slopes, buildings, wooden areas, high farm crops, etc. The alignment of hill roads should ensure clear visibility up to the min : stopping sight distance all along it's  length. It will be Impracticable normally to provide visibility corresponding to the overtaking sight distance. Vision berms should be provided on the inside of horizontal curves The width of these berms should equal the set back distance . While designing for safe visibility distance, it should not be ignored that drivers visual difficulty is magnified by hazy visibility during the monsoons:-- Er Fateh Chand Guleria, RTI welfare Association registered number HPCD 3552 , Bilaspur Himachal Pradesh phone number 9459334377