3.4 SIGHT DISTANCE AND ITS TYPES

  

Sight distance is the actual length of road over which a driver sitting at a specified height in a vehicle can see objects either moving or stationary on the road surface.

In general, the standards for sight distance should satisfy the following conditions:

• Driver travelling at the design speed has sufficient sight distance or road length visible to stop vehicle, in case of any obstruction on the road ahead, without collision.
• Driver travelling ahead should be able to overtake slow moving vehicles, at reasonable intervals, without causing obstruction or hazard to traffic of opposite direction.
• The driver entering a uncontrolled intersection, he should have sufficient visibility to enable him to control his vehicle in order to avoid collision with another vehicle.

In highway engineering sight distance are of following types

1. Stopping sight distance (SSD)
2. Overtaking sight distance (OSD)

1. Stopping sight distance (SSD):

This is defined as the sight distance that is available for the moving the vehicle in the highway that will enable the driver to stop the vehicle safely without collision with any other obstacle. The Stopping distance can be defined as the sum of Lagging distance to the brake distance. For the purpose of measuring the stopping sight distance or visibility ahead. NRS suggested height of the driver’s eye level is 1.2m and the height of the object is 0.15m above the road surface. The sight distance available on a road to a driver at any instant depends on

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

2. Overtaking sight distance (OSD):

The minimum distance available for the driver to safely overtake the slow vehicle in front of him by considering the traffic in the opposite direction is called as the overtaking sight distance. This distance will make us see whether the road is clear to undergo an overtaking movement. The overtaking sight distance is also called as the passing sight distance that will be measured along the center line of the road. This is the line level over which the driver keeping an eye level of 1.2 m above the road level can easily see the top of the object 1.2 m above the road surface.

The main factors that affect the OSD are:

• Spacing Between the vehicles
• Speed of the vehicles
• The gradient of the road
• The acceleration rate of the overtaking vehicle
• The velocities of the vehicle which is overtaking, overtaken and that coming in the opposite direction
• The driver skill
• The reaction of the driver

Factors on which stopping distance depends

• Efficiency of brake
• Friction resistance between the Road and the Tire
• Speed of the Vehicle
• Reaction Time of Driver’s
• Gradient of the Road

• Efficiency of brake: Many factors like the age of the vehicle, the characteristics of the vehicle will affect the brake efficiency of the vehicle. An efficiency of 100% implies that the vehicle will stop at the moment the brake is applied. Obtaining 100% is not practicable. This is an ideal condition of the vehicle. This means, for a lower value of brake efficiency, it is necessary to obtain a higher value of sight distance. In the process of determining a safe geometric design, it is required to assume 50% brake efficiency.
• Friction resistance between the Road and the Tire: The stopping of a vehicle is also dependent on the frictional resistance between the tire and the road. Having a higher value of frictional resistance will result in efficient stopping of the vehicle when applied. Here, the sight distance required will be less. During the computation of sight distance, no special provision or consideration is given for the brake efficiency. This is considered along with the factor of longitudinal friction. In India, the value of longitudinal friction is between 0.35 and 0.4. This is as per the Indian Road Congress.
• Speed of the Vehicle: The speed of the vehicle clearly affects the sight distance. If the speed employed is high, the time required to stop the vehicle will be high. This means that with the increase in the speed there is increase in the sight distance.
• Reaction Time of Driver’s: The time from the moment the driver observes the obstacle in front of him to the moment he applies brake is called as the reaction time of the driver. Based on the PIEV theory, the reaction time can be divided into 4 components. All these times will be combined to form a total perception reaction time while undergoing actual practice and design of highway. From studies conducted it is reviewed that the drivers will require about 1.5 to 2 seconds for normal conditions. As this value may vary based on the characteristics of the vehicle, a higher value for the reaction time can be employed for the design purpose. In India, as per IRC, the reaction time is set to be 2.5 seconds.
• Gradient of the Road: The sight distance is greatly influenced by the gradient of the road. There are chances for the vehicles to stop suddenly when the vehicle is climbing suddenly. This situation will ask for a small sight distance. When the vehicle is moving down, the movement is supported by the gravity action. This will hence ask for more time to stop the vehicle. In this situation, the required sight distance is more.