Stopping Distance (Edexcel IGCSE Physics)

• The stopping distance of a car is defined as:

The stopping distance of a car is the total distance travelled during the time it takes to stop in an emergency

• The stopping distance is the sum of the distance travelled as the driver makes the decision to stop plus the distance travelled as the driver applies the brakes

Stopping distance formula

• The stopping distance is calculated using the following formula:

Stopping distance = Thinking distance + Braking distance

Thinking distance

• Thinking distance is defined as

Thinking distance is the distance travelled in the time it takes the driver to react to an emergency and prepare to stop

• The main factors affecting thinking distance are:
  • The speed of the car
  • The reaction time of the driver 
• The reaction time is defined as:

A measure of how much time passes between seeing something and reacting to it

• The average reaction time of a human is 0.25 s
• Reaction time is increased by:
  • Tiredness
  • Distractions (e.g. using a mobile phone)
  • Intoxication (i.e. consumption of alcohol or drugs)

Braking distance

• Braking distance is defined as

the distance travelled under the braking force in metres (m)

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

Calculating stopping distance

• For a given braking force, the speed of a vehicle determines the size of the stopping distance
• The greater the speed of the vehicle, the larger the stopping distance

The Effect of speed on stopping distance

A vehicle's stopping distance increases with speed. At a speed of 20 mph the stopping distance is 12 m, whereas at 60 mph the stopping distance is 73 m (reproduced from the UK Highway Code) 

A Table Showing Speed and Stopping Distance

Speed (mph)	Speed (m/s)	Stopping Distances (m)
20	9	12
30	14	23
40	18	36
50	22	53
60	27	73

• There are various factors which can affect a vehicle's stopping distance
  • Vehicle speed
    • The greater the speed, the greater the vehicle's braking distance will be
    • This is because the brakes will need to do more work to bring the vehicle to a stop
  • Vehicle mass
    • The more massive the vehicle, the more distance it will travel as it comes to a stop
  • Road conditions
    • Wet or icy roads make the brakes less effective and the vehicle travels further as it comes to a stop
  • Driver reaction time
    • Thinking distance is increased if the driver is distracted, for example by a phone, satnav, radio or a person
    • Thinking distance is increased if the driver is tired, on certain types of medication, or is under the influence of alcohol or drugs