Stopping Sight Distance Calculation on a 2% descending gradient

Stopping sight distance is the distance required for a driver to bring the vehicle to a complete stop after he/she has seen an object on the road. The stopping sight distance is calculated based on various factors such as the speed of the vehicle, the gradient of the road, and the reaction time of the driver.

Formula for calculating stopping sight distance: SSD = (0.278Vt + V^2/2g(μ + f)) + (V^2/2ag)

Where, SSD = Stopping sight distance
V = Speed of the vehicle in km/hr
t = Driver's reaction time in seconds (usually taken as 2 seconds)
μ = Coefficient of friction between the tire and the road surface
f = Grade resistance in percentage
g = Acceleration due to gravity (9.81 m/s^2)
a = Deceleration rate (taken as 4.5 m/s^2 for the worst-case scenario)

Given, descending gradient = 2%
We need to calculate the stopping sight distance for a national highway.

Solution:
Let's assume the speed of the vehicle is 80 km/hr (22.22 m/s) and the driver's reaction time is 2 seconds.

Using the formula,
SSD = (0.278Vt + V^2/2g(μ + f)) + (V^2/2ag)

SSD = (0.278 x 22.22 x 2) + ((22.22^2)/(2 x 9.81 x (0.7 + 2))) + ((22.22^2)/(2 x 4.5 x 9.81))

SSD = 132 meters (approx)

Therefore, the stopping sight distance on a national highway at a descending gradient of 2% is 132 meters. The driver should be able to see the object on the road at least 132 meters ahead to bring the vehicle to a complete stop safely.