SIGHT DISTANCE

SIGHT DISTANCE, Importance, Restrictions to sight distance, Types of sight distance ,Stopping sight distance , safe overtaking intermediate sight distance ,head-light sight distance

1. SIGHT DISTANCE Er . Ramprasad KUMAWAT M,TECH DEPARTMENT OF CIVIL ENGINEERING

2. Sight distance and importance One of the important factors on which safe and efficient operation of vehicle on roads depends is the road length when an obstruction, if any, becomes visible to the driver in the direction of travel. In other words the distance visible ahead to the driver is very important for safe vehicle operation on a highway.

3. `Sight distance' is the length of road visible ahead to the driver at any instance. Sight distance available at any location of the carriageway is the actual distance a driver with his eye level at a specified height above the pavement surface has visibility of any stationary or moving object of specified height which is on the carriageway ahead. The sight distance between the driver and the object is measured along the road surface.

4. Restrictions to sight distance Restrictions to visibility or sight distance may be caused in the following circumstances: (a) At horizontal curves, when the line of sight is obstructed by objects at the inner side of the curve). Here the sight distance is measured along the centre line of the horizontal curve when the vehicle driver is able to see another vehicle or object on the carriageway,

5. (b) At a vertical curve the line of sight is obstructed by the road surface of the summit curve (i.e., a vertical curve of the road with convexity upwards), (c) In this case also the sight distance is measured along the centre line of the vertical curve when the vehicle driver is able to see another vehicle or object on the road.

6. (d) At an uncontrolled intersection when a driver from one of the approach roads is able to sight a vehicle from another approach road proceeding towards .the intersection. Here the sight distance for each vehicle driver is the distance from the position when the two can see each other up to the intersection point of the two roads

7. Fig : Restrictions to sight distance

8. Types of sight distance Sight distance required by drivers applies to both geometric designs of highways and for traffic control. Three types of sight distance situations are considered in the design: (a) Stopping sight distance (SSD) or absolute minimum sight distance (b) Safe overtaking sight distance (OSD) or passing sight distance (c) Safe sight distance for entering into uncontrolled intersections

9. Therefore the following requirements should be taken into account during the design of road geometrics: (a) Safe stopping: Driver travelling at the design speed has sufficient sight distance or length of road visible ahead to stop the vehicle without collision, in case of any obstruction on the road ahead. As safe stopping is most essential requirement to avoid collision, this requirement has to be invariably fulfilled all along the road

10. (b) Safe overtaking: Driver travelling at the design speed should be able to safely overtake the slower vehicles without causing obstruction or hazard to traffic of opposite direction, at reasonable interval

11. (c)Safety at an uncontrolled intersection: Driver entering an uncontrolled intersection has sufficient visibility to enable him to take control of his vehicle and to avoid collision with another vehicle

12. Apart from the three situations mentioned above, the following sight distances are considered by the IRC in highway design: ( i ) Intermediate sight distance: This is defined as twice the stopping sight distance. When overtaking sight distance cannot be provided, intermediate sight distance (ISD) is provided to give limited overtaking opportunities to fast vehicles..

13. (ii) Head-light sight distance: This is the distance visible to a driver during night driving under the illumination of the vehicle head lights. This sight distance is critical at up-gradients and at the ascending stretch of the valley curves.

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