Sight Distance

Sight distance is the length of roadway visible to a driver, it is stationary or moving at a specified height above the carriageway, the distance alongside the middle line of the street, at which a driver has visibility of an object.

Here we will learn about sight distance, types of sight distance & much more.

Introduction to sight distance:

Sight distance is the length of the highway a driver needs to be able to see clearly.

It is important for drivers on a highway to adequately look down the highway to identify an item in the route and react appropriately to avoid an accident.

The sight distance standards must meet the following conditions:

  • Without a collision, in the case of any obstacle on the road ahead, the driver traveling at design speed has sufficient sight distance or road length to appear to stop the vehicle.
  • A driver traveling forward must be able to overtake slow-moving vehicles at reasonable intervals without hindering or threatening traffic in the opposite direction.
  • The driver enters an uncontrolled intersection, must have sufficient visibility so that he can control his vehicle to avoid hitting another vehicle.

Types of sight distance:

1.Stopping or non- passing sight distance (SSD):

The apparent distances a driver needs to stop their vehicle before completing a stationary object on the road is called a stop or non-passing sight distances.

2.Overtaking sight distance (OSD):

The minimum distances open to the driver’s vision on a two-way road pass to enable him to overtake another vehicle with protection against traffic from the opposite direction is called the overtaking sight distances.

3.Intermediate Sight Distance (ISD):

The distance that vehicle drivers attempt a reasonable opportunity to overtake with caution is known as the intermediate vision distance.

4.Lateral Sight Distance (LSD):

The sight distances required by the driver of a vehicle that crosses one other vehicle to the intersection reacts.

It applies to convey its vehicle to a dead stop on the intersection without a collision or accident, it is known as safe sight distances for getting into an intersection or lateral sight distances.

5.Head light sight distances (HSD):

The distances the driver sees when driving under the influence of illuminated headlights of a vehicle is called the headlight sight distances.

Factors affecting sight distance:

1.Speed of the vehicle:

The speed of the vehicle impacts the distance traveled by the driver in complete response time, the higher the speed the larger the distance, this is known as the lag distance.

Similarly, the distance travelled by the driver after the utility of the brake i.e. extra speed could be extra braking distance.

2.gradient of the highway:

The gradient can be positive or negative and the required stopping sight distance can be shorter and higher respectively.

Within the event of an upward (positive) slope, an element of the force of gravity will assist stop the vehicle.

3.Friction between the tyre and the road surface:

The friction between the tire and the highway floor is determined by the kind of highway surface and the situation of the tire, it is decided by the velocity of the vehicle.

The more friction, the shorter sight distances are likely to be required, however, when the friction is much less, will probably be larger.

4.Total reaction time of the driver:

It is the time from the instance the obstruction is seen to the driver to the instance when he effectively applies the break.

In complete response, the time the vehicle strikes at the speed at which the driver is transferring or taken because of the design speed.

Therefore, if the overall response time of the driver is longer, the larger the distance travelled, the larger the stopping sight vision distance.

5.Brake Efficiency:

The brake efficiency means the tire winding is completely closed, but this will certainly result in the skidding of the vehicle.

The efficiency of the brake is considered by reducing the original value of friction in the range of 0.35 to 0.40.

Also read: Street Lighting, Town Planning & Road Pattern


The sight distance is the length of the road visible to the driver at any given time.

The distances a driver receives in an overnight trip is known as the headlight sight distances; at night the drive is facilitated by the lighting of the headlights.

The visible distance available to enter an intersection is called the safe vision distance.

Hello, I'm Rahul Patil founder of, I had studied B.E. Civil. This blog provides authentic information regarding civil structures, equipment, materials, tests & much more.

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