Stopping Distance

Definition of Stopping Distance

Stopping distance is the total distance a vehicle travels from the moment the driver recognises a hazard to when the vehicle comes to a complete stop. It is the sum of two parts:

• Thinking distance: the distance travelled during the driver's reaction time
• Braking distance: the distance travelled while the brakes are applied and the vehicle is slowing down

Stopping distance depends on both how quickly the driver reacts and how effectively the vehicle can be slowed down.

Thinking Distance

Thinking distance is the distance a vehicle travels during the driver's reaction time — the time taken to realise the need to stop and to move the foot to the brake pedal.

This distance depends on the speed of the vehicle and the driver's alertness. If the driver is tired, distracted (e.g. using a phone), or under the influence of alcohol or drugs, reaction time increases, so thinking distance increases.

For example, if a car is travelling at 20 m/s and the driver's reaction time is 0.75 seconds, the thinking distance is:

$\text{Thinking distance} = \text{speed} \times \text{reaction time} = 20 \times 0.75 = 15 \text{ m}$

Example calculation: If a car travels at 10 m/s and the driver’s reaction time is 1 second, the thinking distance is $10 \times 1 = 10$ m.

Braking Distance

Braking distance is the distance a vehicle travels from the time the brakes are applied until it stops completely.

It depends on:

• The vehicle's speed: higher speeds mean longer braking distances
• Road conditions: wet, icy, or uneven roads increase braking distance
• Vehicle condition: worn brakes or tyres reduce braking efficiency and increase braking distance
• Weather conditions: rain, snow, or ice reduce friction between tyres and road, increasing braking distance

For example, a car travelling at 30 m/s on a dry road might have a braking distance of 45 m, but on a wet road, this could increase significantly.

Factors Affecting Stopping Distance

Several factors influence the overall stopping distance:

• Speed: The faster the vehicle travels, the longer both thinking and braking distances become. Stopping distance increases significantly with speed.
• Road surface and weather: Slippery surfaces caused by rain, ice, or oil reduce tyre grip, increasing braking distance.
• Driver condition: Fatigue, distractions, alcohol, or drugs increase reaction time, thus increasing thinking distance.

For example, if a driver is tired, their reaction time might increase from 0.75 seconds to 1.5 seconds, doubling the thinking distance at the same speed.

Example: Calculating Total Stopping Distance

A driver is travelling at 25 m/s. Their reaction time is 1 second. The braking distance on a dry road is 50 m. Calculate the total stopping distance.

Thinking distance = speed × reaction time = $25 \times 1 = 25 \text{ m}$

Braking distance = 50 m (given)

Total stopping distance = thinking distance + braking distance = $25 + 50 = 75 \text{ m}$