GEOMETRICAL DESIGN STANDARDS REPRESENT AN ACCEPTABLE COMPROMISE BETWEEN THE ROAD USERS REQUIREMENTS AND THE CONSTRUCTION COST

Sight distance or 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 the expected speed of the vehicles and the breaking distance required to stop and which should also include the driver's 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 sight 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 can not be provided, warning boards should be fixed for reducing the speed. The eye level of drivers can vary from 1.05 m above the carriage way in a standard car to approximately 2m in large commercial vehicles. For the purpose of measuring the stopping sight distance or visibility ahead, IRC has suggested the height of eye level of driver as 1.2 metres and the object height is 0.15 metre above the road surface. Hence stopping distance at a summit curve is that distance measured along the road surface at which an object of height 0.15 metre can be seen by a driver whose eye is at a height of 1.2 metres above the road surface. Sight distance both vertical and horizontal, should be measured along the eye level (1.2 m) above the carriage way along the centre lines of both the rear side and outside lanes of the carriage way. 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. For more clarifications related to Stopping sight distance and overtaking (or passing) Sight distance for various speeds for rural Highways IRC :66 may be verified for design criteria, however absolute minimum sight distance is the safe stopping sight distance Er Fateh chand Guleria, RTI Welfare Association registered number HPCD, 3552 , Bilaspur Himachal Pradesh phone number 9459334377