Stopping sight distance is the minimum distance required for the driver to stop a vehicle moving at design speed, without colliding with an obstruction. Stopping sight distance comprises two parts, namely, lag distance and braking distance. Detailed information on stopping sight distance will be covered in this blog.
Sight Distance
Before getting into stopping sight distance it is essential to know about sight distance. Sight distance is the length of road visible for a driver at any given instant. It is essential to maintain appropriate sight distance in every cross-section of the road.
According to IRC, the eye level of the driver is 1.2 metres and the height of the obstruction is 0.15 metres. This is considered standard across all cases.
Factors Affecting Stopping Sight Distance
As said, stopping sight distance is the minimum distance required for the driver to stop a vehicle moving at design speed, without colliding with an obstruction. It depends on five factors as mentioned below.
Design speed
Reaction time
Co-efficient of longitudinal friction
Longitudinal slope
The efficiency of the brakes
We will see all of these terms in detail while deriving the equation for stopping sight distance.
Derivation for Stopping Sight Distance
Stopping sight distance has two parts, namely, lag distance and brake distance. Each of these terms is explained and the stopping sight distance formula is derived further.
1) Lag Distance
Lag distance is the distance travelled by a vehicle during the reaction time of the driver.
Lag distance = Design speed * Reaction time
The reaction time of the driver is the time required by the driver to analyse the situation and apply the brake. It is based on PIEV theory, i.e., Perception, Intellection, Emotion, and Volition. It is generally taken as 2.5 seconds.
2) Brake Distance
Brake distance is the distance travelled by the vehicle after the application of the brake and the instance of its stopping. It is dependent on the coefficient of longitudinal friction, the gradient of the road, and the efficiency of the brake.
The coefficient of longitudinal friction is the frictional resistance offered by the road. It is dependent on the design speed or the velocity of the vehicle.
Design speed (in kmph) | Coefficient of longitudinal friction (f) |
20 to 30 | 0.4 |
30 to 40 | 0.38 |
40 to 50 | 0.37 |
50 to 65 | 0.36 |
65 to 80 | 0.35 |
80 to 100 | 0.34 |
Braking distance (Lbrake) in a levelled road is found by equating the kinetic energy of the vehicle with the frictional resistance offered by the road.
1/2 * m * v^2 = Ff * L(brake)
1/2 * m * v^2 = f * m*g * L
L = (v^2) / (2*g*f)
Similarly for road surfaces with rising gradients,
L = (v^2) / (2*g*(f+n)), where n is the slope in fraction
Similarly for road surfaces with falling gradients,
L = (v^2) / (2*g*(f-n)), where n is the slope in fraction
Stopping Sight Distance Formula
Combining both lag distance and brake distance, we get, the stopping sight distance formula as,
SSD = v * t + ((v^2) / (2*g*(f ± n))),
To put it in a better way,
SSD = 0.278*v*t + ((v^2) / (254 * (f ± n))),
where,
v - design speed in kmph
t - reaction time of the driver (2.5 seconds)
The same is provided in a pictorial format for better understanding.
Note:
SSD for 2 lane road is simply SSD. But, SSD for single lane roads with two-way traffic is twice the SSD, i.e., SSD = 2*SSD.
Based on stopping sight distance several other important factors could be found as mentioned below.
Head-on Collision Distance
It is the addition of two stopping distances based on the speeds of two different vehicles.
Head-on collision distance = SSD1 + SSD2
Head Light Distance
Headlight distance is the same as SSD.
Intermediate Sight Distance
This is essential when two-way traffic in a single lane road exists. It is essential to provide limited overtaking opportunities where Overtaking Sight Distance is not possible.
Intermediate Sight Distance = 2 * SSD
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