Sight distance | What is sight distance?

Sight distance | What is sight distance?

The driver’s ability to look ahead contributes to the safe and efficient operation of the road. Ideally, the geometric design should at all times ensure that any object on the pavement surface is visible to the driver at a normal eye-sight distance. However,

sight distance

If not facilitated. Due to topographic and other constraints, this is usually not feasible, so it is
It is necessary to design roads that are short but safe, visually distant.
sight distance

There are three different visual distances of interest in geometric design:

• stopping sight distance;
• meeting sight distance;
• passing sight distance.


Stopping sight distance

The stopping sight distance comprises two elements:

d1=the distance moved from the instant the object is sighted to the moment the brakes are applied (the perception and brake reaction time referred to as the total reaction time) and

d2= the distance traversed while braking (the braking distance).

Example Stopping Sight Distance Study

Cottonwood Glen may be undergoing a new development near the established residential district,
Including multiple families and many children at present. The proposed development is a new city
Proposed to be along the roadway directly from the park and residential area.

There is no established traffic control and no intersections in this area. At public meetings, parents have raised concerns about the safety of their children crossing the roadway into the park.

This roadway has a vertical curve which can affect the distance of the stopping sight.
Cottonwood Glenn conducted a study on Washington Avenue at 10:00 am on Thursday. The speed limit posted on this roadway is 25 mph. Stopping distance recommended from

stopping sight distance

Reaction time

Total reaction time includes the physical and mental characteristics of the driver, the appearance of the environment, the types and conditions of the road, and the distance, size, color and
The shape of risk.

When drivers are looking intensively at high traffic intensity urban conditions, the response time may be in the range of 0.5-1.0 seconds but the driver response time is usually 2-4 seconds for normal driving in rural conditions.

Overseas Road Note 6 assumes a total reaction time of 2 sec., while the SATCC Recommendations are using 2.5 sec.

d1 The distance traveled before the brakes are applied is:


d1=total reaction distance in m;
V=initial vehicle speed in kmh−1;
t=reaction time in sec.
d2  The braking distance, d2, is dependent on vehicle condition and characteristics, the coefficient of friction between tire and road surface, the gradient of the road, and the initial vehicle speed.


d2 = braking distance in metres;
V = initial vehicle speed in km h−1;
f = coefficient of longitudinal friction;
g = gradient (%; positive if uphill and negative if downhill).

The determination of longitudinal friction, f design values ​​is complicated due to many factors involved. However, it is well known that F-values ​​are decreasing for higher vehicle speeds on wet roads.

The longitudinal collision design values ​​used in the Overseas Road Note 6 and SATCC recommendations are shown in Table 7.3. Calculated stop vision distances (d + d) are listed in Tables 7.4 and 7.5.

Meeting sight distance

Meeting vision distance is the distance required by drivers of two vehicles moving in opposite directions to bring their vehicles to a safe stop after they have seen each other.

The visit distance is usually calculated twice the minimum stopping distance. For single-carriageways, it is desirable to meet the entire distance of the road, and if the width of the carriageway is less than 5.0 m, it is generally necessary to pass safely on such narrow roads. Speed.

Passing sight distance

The judgments of overtaking are the factors that affect the distance of overtaking (overtaking) vision Drivers, speed and size of overtaken vehicles, acceleration capabilities of overtaking vehicles, and speed of oncoming vehicles.

sight distance



Driver’s judgment and behaviour are important factors that vary considerably among drivers. For design purposes, the selected passing distance must be adequate for most people Drivers.

Passing sight distance is determined experimentally and is usually based on the needs of a passenger car.

Heavy commercial vehicles require more time than cars, but on the other hand, commercial motorists have longer visibility due to their higher eye height.

They are therefore able to judge quickly and well, so that the gap can determine whether or not the overtaking is appropriate, thus partially offsetting any additional overtaking length required.

There are considerable differences in the various criteria for passing the distance of sight
For differing assumptions about the gaps in the unit of passing proficiency It is divisible, maneuverable, and, to a lesser extent, different predicted speeds for the driver Behavior.

Overseas Road Note 6 and The SATCC recommendations are shown in Table 7.6.

The passing sight distances recommended for use by the SATCC countries are based on a speed difference of 20kmh between the passing vehicle and the overtaken vehicle and acceleration rates proposed by AASHTO.

The reduced passing distance is based on the assumption that the overtaking vehicle is more likely to repent at an early stage and return to a slower-moving vehicle.

The overtaking sight distances recommended by Overseas Road Note 6 also assume that the overtaking vehicle may safely abandon the manoeuvre if an approaching vehicle comes into view. Other design criteria are not specified.

You can also watch this for a better understanding:

Intersection sight distance

The distance from the intersection is the approaching distance along the main road. The vehicle must be seen to allow the vehicle to cross over the far intersection road Or securely merge with road traffic.

The distance to the intersection depends on the design speed, the width of the crossing road, and the characteristics of the vehicle crossing or merging into the main road. Generally, the minimum incision distance can be taken


S = sight distance along the main road in meters;
V = design speed in kmh−1.

Also Read:

  1. Modes Of Transportation
  2. Horizontal Alignment Of Road
  3. MORTH




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