Advertise with us
Estimating Sheet

How to work out stopping sight distance in highway

Construction Software

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

How to work out stopping sight distance in highway