Stopping distance and reaction time

The stopping distance of a vehicle is the distance it will travel before decelerating to a stop. It is the sum of the thinking distance and braking distance.
- The thinking distance will be the distance travelled before the driver reacts to the need to stop. Thinking distance is changed by conditions affecting the driver while braking distance is affected by conditions affecting the vehicle or road.
- The braking distance will be the distance the vehicle travels after its brakes are applied. The braking distance of a vehicle will increase if:
  - The roads are slippery, icy, smooth or other conditions that will reduce the friction it causes.
  - The braking force the vehicle can provide is reduced.
  - The vehicle is not maintained well, particularly its tyres and brakes.
The greater the speed of a vehicle, the further its stopping distance will be.
People’s reaction times will vary. They are typically between 0.2s and 0.9s.
Reaction times can be tested by asking someone to grab a ruler as it is dropped or press a timed button after a light comes on.
A driver’s reaction time will be increased by drinking alcohol, drugs and certain medications, tiredness, or becoming distracted (e.g. using a phone whilst driving).

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