Stopping distance and stopping sight distance

Stopping distance is the total distance travelled by a vehicle from the instant the driver sights an object or hazard ahead until the vehicle comes to a complete stop. It is the sum of the lag distance (travelled during the reaction time) and the braking distance:

$$SD = v\,t + \frac{v^2}{2g\,(f \pm G)}$$

Factors affecting stopping distance

1. Speed of the vehicle.
2. Reaction time (PIEV) of the driver.
3. Coefficient of longitudinal friction (type and condition of road surface and tyres, wet or dry).
4. Gradient of the road.
5. Brake efficiency and the weight/loading of the vehicle.
6. Weather and visibility, driver's alertness (also fatigue, alcohol).

Relation between stopping distance and stopping sight distance

• The stopping sight distance (SSD) is the clear length of road ahead that the road must give so that the driver can see an object and stop in time.
• It is equal to the stopping distance computed for the design speed with the design values of $t = 2.5$ s and $f$: so the design SSD is simply the stopping distance: $$SSD = 0.278\,V\,t + \frac{V^2}{254\,(f \pm 0.01n)}$$
• The available sight distance at any point of a horizontal or vertical curve (measured with a driver eye height 1.2 m and an object height 0.15 m) must be not less than the stopping distance.
• If the available sight distance is less than the stopping distance, the vehicle cannot stop before the object, so the road geometry (curve length, set-back, speed limit) is revised.