1. Sight Distances

2. Sight Distances
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 driver from some distance ahead . This distance is said to be the sight distance.

3. Types of sight distance
Sight distance available from a point is the actual distance along the road surface, over which a driver from a specified height above the carriage way has visibility of stationary or moving objects. sight distance situations considered for design are:
Stopping sight distance (SSD) or the absolute minimum sight distance
Intermediate sight distance (ISD) is the defined as twice SSD
Overtaking sight distance (OSD) for safe overtaking operation
Head light sight distance is the distance visible to a driver during night driving under the illumination of head light

4. Factors The computation of sight distance depends on:
Reaction time of the driver
Speed of the vehicle
Efficiency of brakes
Frictional resistance between the tire and the road
Gradient of the road

5. Reaction time of the driver
Reaction time of a driver is the time taken from the instant the object is visible to the driver to the instant when the brakes are applied.
Many of the studies shows that drivers
require about 1.5 to 2 seconds under normal conditions.
However taking into consideration the variability of driver characteristics, a higher value is normally used in design

6. Speed of the vehicle
The speed of the vehicle very much affects the sight distance.
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.

7. Efficiency of brakes
The efficiency of the 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 it could be understood that sight distance required will be more when the efficiency of brakes are less.
Also for safe geometric design, we assume that the vehicles have only 50% brake efficiency.

8. Frictional resistance between the tire and the road
The frictional resistance between the tire and road plays an important role to bring the vehicle to stop.
When the frictional resistance is more, the vehicles stop immediately. Thus sight required will be less.
The value of longitudinal friction in between to 0.4.

9. Gradient of the road
Gradient of the road also affects the sight distance. 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 to stop the vehicle. Sight distance required will be more in that case.

10. Stopping sight distance
SSD is the minimum sight distance available on a highway at any spot having sufficient length to enable the driver to stop a vehicle traveling at design speed, safely without collision with any other obstruction.
SSD is made up of two components:
Braking Distance and
Perception-Reaction Time

12. Criteria for Sight Distance
Driver eye height: for passenger vehicle’s = 3.5 feet above surface
Height of object in roadway = 2 feet (SSD)
Height of opposing vehicle = 3.5 feet (PSD)

13. deceleration rate of 11.2 feet/second2

14. PASSING SIGHT DISTANCE
Passing sight distance considerations are limited to 2-lane, 2-way highways. On these facilities, vehicles may overtake slower moving vehicles.

15. Cont..,
Passing sight distance for passenger cars is measured from a 3.5 feet (1.080 m) height of eye to a 3.5 feet (1.080 m) height of object.
The 3.5 feet (1.080 m) height of object allows the opposing driver to see the top of a typical passenger car.
The recommended value of truck driver eye height for design is 7.6 feet (2.330 m) above the roadway surface.

16. INTERSECTION SIGHT DISTANCE (ISD)
In general, ISD refers to the corner sight distance available in intersection quadrants that allows a driver approaching an intersection to observe the actions of vehicles on the crossing leg(s).

17. Cont..,
The height of eye for passenger cars is assumed to be 3.5 ft (1.080 m) above the surface of the road.
The height of object (approaching vehicle on the major road) is also assumed to be 3.5 ft (1.080 m).
An object height of 3.5 ft (1.080 m) assumes that a sufficient portion of the oncoming vehicle must be visible to identify it as an object of concern by the minor road driver

18. Cont..,
The sight distance should be provided such that the drivers on either side should be able to see each other.
Design of sight distance at intersections may be used on three possible conditions:
Enabling approaching vehicle to change the speed
Enabling approaching vehicle to stop
Enabling stopped vehicle to cross a main road

20. Decision Sight Distance
The available sight distance on a highway needed to detect an unexpected or otherwise difficult-to-perceive information in a roadway environment

21. Cont..,
Decision sight distance values are greater than stopping sight distance values because they give the driver an additional margin for error and afford sufficient length to maneuver at the same or reduced speed rather than to just stop

22. Cont..,
Consider decision sight distances at locations where there is high likelihood for driver error in information reception, decision making, or control actions.
If site characteristics and budget allow, locate these highway features where decision sight distance can be provided. If this is not practicable, use suitable traffic control devices and positive guidance to give advanced warning of the conditions.