Stopping Sight Distance Calculation

In this civil engineering article, you will learn the calculation process for sight distance in a highway as highway geometric design.

Sight distance belongs to the distance of highway a driver requires that can be clearly visible to a driver.

The sight distance is also described as the length of road or street that is far ahead to be viewed by the driver.

While making design of a highway or road, the following types of sight distances are commonly found :-

1) Stopping sight distance
2) Overtaking sight distance
3) Intersection sight distance

Stopping sight distance – Stopping sight distance is the sum of reaction and braking distance.

It is defined as follow :-
D = D1 + D2
Here D denotes stopping, D1 reaction sight distance and D2 is the braking distance.

The value of D1 and D2 can be measured with the following formula :-

D1 = 0.28Vt
D2 = 0.01V²
or, D2 = (0.28V)² / 2gɳ
V refers to the speed or velocity of the vehicle. It is taken as design speed in KM/hrs.
T refers to the time in seconds.
G refers to the gravitational acceleration.
ɳ refers to the brake efficiency.
S refers to the road gradient.

Also Read: How to measure different types of curves in roads

The braking sight can also be measured with another formula given below :-
D2 = V2 / 100 ± 2.5 S; S denotes the gradient of road.

To clearly explain it, the solution is given for the following problem :-

Calculate the stopping sight distance of a highway for which the design speed is 65 km/hrs. Suppose, the brake efficiently is 40% and the driver reaction time is 3 seconds.

We know, stopping sight distance = Reaction distance + braking distance
D = D1 + D2
D = 0.28Vt + (0.28 V)² / 2gɳ
D = 0.28(65)(3) + (0.28 x 65)² / (2)(9.81)(0.40)[velocity = 65 km/hour; t = 3 seconds; brake efficiency = 40%]
D = 54.6 + 42.2 = 96.8 meters
So, 96.8 meters should always be noticeable to the driver for risk-free travel.

Video Source: SL Khan