Recall that the stopping distance of a vehicle is made up of the sum of…

The idea

The stopping distance of a vehicle is the total distance travelled from the moment the driver sees a hazard until the vehicle comes to rest. It is the sum of two parts. Thinking distance is the distance covered while the driver reacts before pressing the brake, so it depends on reaction time and speed. Braking distance is the distance covered while the brakes slow the vehicle to a stop, so it depends on speed, road conditions and the condition of the brakes and tyres. The relationship is written as stopping distance = thinking distance + braking distance, and each part increases with speed.

What earns the marks

• States that stopping distance equals thinking distance plus braking distance.
• Defines thinking distance as the distance travelled during the driver's reaction time.
• Defines braking distance as the distance travelled while the vehicle slows to a stop.
• Explains that both thinking distance and braking distance increase as speed increases.
• Links thinking distance to reaction time and braking distance to braking force or road conditions.

Common mistakes and how to put them right

• Writes stopping distance as thinking distance minus braking distance.

  Put it right: The two distances add together.

• Treats thinking distance as the distance covered while braking.

  Put it right: It is covered before the brakes act.

• Assumes braking distance is unaffected by speed.

  Put it right: It rises steeply as speed increases.

• Ignores driver reaction time when explaining thinking distance.

  Put it right: Reaction time sets how far the vehicle travels before braking.

Exam tips

• Learn the word equation stopping distance = thinking distance + braking distance exactly.
• When asked to compare the two parts, say what each depends on rather than only naming them.
• Use the idea that both parts grow with speed to explain why faster vehicles need much longer to stop.