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 specify height above the pavement surface has visibility of any stationary or moving object of specify height which is on the carriageway ahead. The sight distance between the driver and the object is measure along the road surface.

Restrictions to Sight distance

• At horizontal curves, when the line of sight is obstruct by object at the inner side of the curve. Here the sight distance is measure along the centre line of the horizontal curve when the vehicle driver is able to see another vehicle or object on the carriage way.

• At a vertical curve the line of sight is obstruct by the road surface of the summit curve ( i.e., a vertical curve of the road with convexity upward ).

• In this case also the sight distance is measure along the centre line of the vertical curve when the vehicle driver is able to see another vehicle or object on the road.
• At an uncontrolled intersection when a driver from one of the approach is able to sight to 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 an see each other up to the intersection point of the two roads.

Types of sight distance

Sight distance require by drivers applies to both geometric designs of highways and for traffic control. Three types of sight distance situations are consider in the design:

• Stopping sight distance (SSD) or absolute minimum sight distance.
• Safe overtaking sight distance (OSD) or passing sight distance.
• Safe sight distance for entering into controlled intersections.

Important points remember during the design of road geometric :

Safe Stopping

Driver travelling at the design speed has sufficient sight distance. Also length of road visible ahead to stop the vehicle without collision. Whenever, in case of any obstruction on the road ahead. As safe stopping is most essential requirement to avoid collision. So, this requirement has to be invariably fulfil all along the road.

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 intervals.

Safety at an uncontrolled intersection

Driver entering an uncontrolled intersection has sufficient visibility to enable him to take control of his vehicle. To avoid collision with another vehicle.

Apart from the three situations mention above, the following sight distances are consider in highway design :-

Intermediate sight distance

It is twice the stopping sight distance. When overtaking sight distance cannot be provide, intermediate sight distance (ISD) is provide to give limit overtake opportunities to fast vehicles.

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.

Stopping Sight Distance

The minimum distance visible to a driver ahead or the ‘sight distance’ available on a highway at any spot should be of sufficient length to safely stop a vehicle travelling at desire speed, without collision with any other obstruction. Therefore this stopping sight distance (SSD) is also refer as absolute minimum sight distance. This is also some times termed as ‘non passing sight distance’.

The sight distance available to a driver travelling on a road at any instance depends on the following factors :

• Features of the road ahead
• Height of the driver’s eye above the road surface
• Height of the object above the road surface.

Features of the road ahead

The features of the road ahead which affect the sight distance are the horizontal alignment and vertical profile of the road, the traffic condition and the position of obstructions.

Height of the driver’s eye above the road surface

At vertical summit curves the height of the driver’s eye and the object above road level are more important factors affecting the visibility.  

Height of the object above the road surface

The height of an object to be considered for stopping a vehicle depends on what might be a source of danger to the moving vehicle. 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 m and the height of the object as 0.15 m above the road surface.

Hence the stopping distance available at a summit curve is the distance measured along the road surface at which an object of height 0.15 m can be seen by a driver whose eye is at a height of 1.2 m above the road surface.

Factors on which stopping distance depends

The distance within which a motor vehicle can be stopped depends upon the factors listed below:

• Total reaction time of the driver
• Speed of vehicle
• Efficiency of brakes
• Frictional resistance between the road and the tyres and
• Gradient of the road, if any

Total reaction time of driver

Reaction time of the driver is the time taking from the instant the object is visible to the driver to the instant the brakes are effectively apply. The actual time gap or the reaction time of the driver depends on several factors. During this period of time the vehicle travels a certain distance at the original speed, which may be assuming to be the design speed of the road. Thus the stopping distance increases with increase in reaction time of the driver. The total reaction time, t may be split up into two parts:

• perception time 
• brake reaction time.

Perception time 

The ‘perception time’ is the time required for a driver to realise that brakes must be applied. It is the time from the instant the object comes on the line of sight of the driver to the instant he realises that the vehicle needs to be stopped. The perception time varies from driver to driver and also depends on several other factors such as the distance of object and other environmental conditions.

Brake reaction time

The ‘brake reaction time’ also depends on several factors including the skill of the driver. This type of the problems and various other environmental factors.

Generally total reaction time of the driver is taking together, as this is found to vary considerably depending on several factors. The total reaction time may be explain with the help of ‘PIEV’ theory.

‘PIEV’ theory

According to ‘PIEV’ theory, the total reaction time of the driver is split into four parts, viz., time taking by the driver for:

• Perception,
• Intellection
• Emotion
• Volition

Perception time

‘Perception time’ is the time requires for the sensations receiving by the eyes or ears of the driver to be transmitting to the brain through the nervous system and spinal cord. In other words, it is the time requires to perceive an object or situation.

Intellection time

Intellection time is the time requires for the driver to understand the situation. It is also the time requires for comparing the different thoughts. Regrouping and registering new sensations.

Emotion time

Emotion time is the time elapse during emotional sensations. As well as other mental disturbance such as fear, anger or any other emotional feelings like superstition etc. with reference to the situation. The emotion time varies for different drivers, but even for a particular driver the emotion time is likely to vary considerably depending upon the situation or the actual problem involve.

Volition time

‘Volition time’ is the time taking by the driver for the final action, such as brake application.

It is possible that the driver may apply brakes. To take any other avoiding action like turning, by the ‘reflex action’, without the normal thinking process,’ which is probably the minimum time for taking a preventive action like brake application.

Speed vehicle

The stopping distance depends very much on the speed of the vehicle. First, during the total reaction time of the driver the distance moved by the vehicle will depend on the speed. Second, the braking distance or the distance moved by the vehicle after applying the brakes, before coming to a stop depends also on the initial speed of the vehicle. Hence it is evident that higher the speed, higher will be the stopping distance.

Efficiency of brakes

The braking efficiency is said to be 100% if the wheels are fully locked preventing them from rotating on application of the brakes. This will result in 100 percent skidding which is normally undesirable, except in utmost emergency. Also skidding is considering to be dangerous. As it is not possible for the driver to easily control a vehicle after it starts skidding. Hence to avoid skid, the braking forces should not exceed the frictional force between the wheels and tyres.