Stopping Distances (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Thinking & Braking Distances

• The stopping distance of a car is defined as:

  The total distance travelled during the time it takes for a car to stop in response to some emergency

• It can be written as an equation involving two distances:

Stopping distance = Thinking distance + Braking distance

• Where:

  • Thinking distance = the distance travelled in the time it takes the driver to react (reaction time) in metres (m)

  • Braking distance = the distance travelled under the braking force in metres (m)

  • Stopping distance = the sum of the thinking distance and braking distance, in metres (m)

• For a given braking force, the greater the speed of the vehicle, the greater the stopping distance

Factors Affecting Stopping Distance

• The thinking distance is defined as:

  The distance travelled by a car from when a driver realises they need to brake to when they apply the brakes

• The reaction distance is equal to:

Reaction Distance = Speed of the car × Driver’s reaction time

Factors affecting thinking distance

• The main factor that affects the thinking distance is the car’s speed, however additional factors can affect the thinking distance

• It is increased by:

  • Tiredness

  • Distractions (e.g. using a mobile phone)

  • Intoxication (i.e. consumption of alcohol or drugs)

• Since these factors can affect the driver's reaction time, they directly affect the thinking distance

Factors affecting braking distance

• The braking distance is the distance a vehicle travels from when the brakes are applied until the vehicle stops

• The main factor affecting braking distance is the vehicle's speed

  • The greater the speed, the greater the braking distance

• Additional factors that increase the braking distance include:

  • Adverse road conditions: wet or icy roads reduce friction between the tyres and the road surface

  • Poor tyre condition: worn tyres have reduced grip, decreasing the friction force available

  • Poor brake condition: worn or faulty brakes reduce the braking force that can be applied