Factors Affecting Stopping Distance

Section 1 of 7Stopping distance

1Hazard seen

• Hazard seen The driver notices a danger and the car is still moving at full speed.

2Thinking distance

• Thinking distance Distance travelled during the driver's reaction time, before the brakes are applied: thinking distance = speed × reaction time.

3Braking distance

• Braking distance Distance travelled once the brakes are applied, while the car slows to rest.

4Stopping distance

• Stopping distance Stopping distance = thinking distance + braking distance.

Section 2 of 7Factors affecting thinking distance

Speed

• Speed At constant reaction time, doubling the speed doubles the thinking distance: the two are directly proportional.

Tiredness

• Tiredness A tired driver reacts more slowly, so travels further before braking.

Distractions

• Distractions Using a phone or talking to passengers lengthens the reaction time.

Drugs and alcohol

• Drugs and alcohol These slow the nervous system and greatly increase the reaction time.

Section 3 of 7Factors affecting braking distance

Factor	Effect	Reason
Higher speed	Much longer braking distance	More kinetic energy must be removed by the brakes
Wet or icy road	Longer braking distance	Less friction between tyres and road, so a smaller braking force before skidding
Worn tyres	Longer braking distance	Less grip, especially in wet conditions
Worn or faulty brakes	Longer braking distance	Smaller braking force is produced
Greater vehicle mass	Longer braking distance	More kinetic energy at the same speed, and more inertia

Section 4 of 7Why speed matters so much

1

A moving car has kinetic energy: E_k = ½ × m × v².

2

To stop, the brakes do work equal to this kinetic energy, transferring it to the thermal store of the brakes and surroundings.

3

Work done = braking force × braking distance, so braking distance = kinetic energy ÷ braking force.

4

Kinetic energy depends on v², so braking distance is proportional to speed squared.

5

Doubling the speed therefore multiplies the braking distance by 4, and trebling it multiplies by 9.

Section 5 of 7Large decelerations are dangerous

Force on passengers

• Force on passengers A very short stopping distance means a large deceleration and so a large force on the car and its occupants.

Brake overheating

• Brake overheating Heavy braking transfers energy to the brakes so quickly that they can overheat and lose effectiveness.

Section 6 of 7Key terms

Term	Meaning
Thinking distance	The distance a vehicle travels during the driver's reaction time.
Braking distance	The distance a vehicle travels while the brakes slow it to a stop.
Stopping distance	Thinking distance plus braking distance.
Friction	A force that opposes motion between surfaces in contact.
Kinetic energy	The energy of a moving object, ½mv2.

Section 7 of 7Exam technique

1

Saying speed only affects braking distanceit affects both thinking distance and braking distance.

2

Writing that doubling speed doubles braking distance: it is a factor of four.

3

Putting tiredness or distraction under braking distance: they change reaction time, so they change the thinking distance.

4

Forgetting that the road surface changes friction between the tyre and road, not the driver's reaction.