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Science

Forces and motion

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Thinking, braking and stopping distance

To be a safe driver you need to understand the factors that affect a car's stopping distance.

The stopping distance depends on two factors:

  • Thinking distance - It takes time for a driver to react to a situation. During this reaction time the car carries on moving. The thinking distance is the distance travelled in between the driver realising he needs to brake and actually braking.
  • Braking distance - The braking distance is the distance taken to stop once the brakes are applied.

stopping distance = thinking distance + braking distance

Factors that might increase stopping distance

Thinking distance can be increased by:

  • greater speed
  • tiredness
  • alcohol and drugs
  • distractions

Braking distance can be increased by:

  • greater speed
  • poor road conditions (icy, wet)
  • car conditions (bald tyres, poor brakes, full of people)

How speed affects stopping distance

a car moving at a  speed of 20 miles an hour would have a thinking distance of 7 miles, and a braking distance of 7 miles. A car moving at 70 miles per hour would have a thinking distance of 22 miles, and a braking distance of nearly 80 miles.

Stopping distances

The diagram shows how, under normal driving conditions, thinking distance and braking distance depend on the speed of the car.

The stopping distance is much further for faster speeds, which is why you should:

  • Keep your distance from the car in front, especially if the road conditions are poor.
  • Keep to the speed limit.

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