Stopping Distances
How long does it really take to stop a car? Split stopping distance into thinking and braking, learn what makes each one grow, and see why doubling your speed does far worse than double the danger.
Get the method right under pressure
Free interactive practice on the steps that lose marks under exam pressure.
Start revising freeWhat you'll cover
Stopping Distances
When a hazard appears, a car cannot stop instantly. First the driver has to react, and then the brakes have to do their work. The total distance covered from spotting the danger to standing still is the stopping distance. It is bigger than most people think, and it grows with speed in a way that catches people out, because its two halves do not grow at the same rate.
Thinking and braking
stopping distance = thinking distance + braking distance. Two halves, and almost every question in this topic turns on not muddling them:
Total stopping distance
A car has a thinking distance of 12 m and a braking distance of 24 m. What is its total stopping distance, in metres (m)?
What lengthens the BRAKING distance?
Select ALL THREE things that increase the braking distance specifically, rather than the thinking distance.
- The same van driven with a full load rather than empty
- Tyres worn down close to the legal tread limit
- A road surface covered in wet leaves
- A driver who has been awake for twenty hours
- A driver reading a text message
- A newly resurfaced, dry road
Which half grows faster?
A car doubles its speed. Thinking distance grows in proportion to speed; braking distance grows with speed squared. What happens to the two halves of the stopping distance?
- Thinking distance roughly doubles while braking distance roughly quadruples, so braking becomes a much larger share of the total and the total more than doubles
- Both roughly double, so the total distance doubles as well
- Thinking distance quadruples while braking distance doubles, so thinking becomes the larger share
- Both quadruple, so the whole stopping distance is four times longer
What the numbers actually are
The Highway Code publishes typical stopping distances for an average car on a dry road. Read the two columns down, not across: • 20 mph: 6 m thinking + 6 m braking = 12 m • 30 mph: 9 + 14 = 23 m • 40 mph: 12 + 24 = 36 m • 50 mph: 15 + 38 = 53 m • 60 mph: 18 + 55 = 73 m • 70 mph: 21 + 75 = 96 m The thinking column climbs 6, 9, 12, 15, 18, 21: dead straight. The braking column climbs 6, 14, 24, 38, 55, 75, and by 70 mph it is more than three times the thinking distance. That gap is the squared term, in real numbers.
How far is 96 metres?
At 70 mph the typical stopping distance is 96 m. Taking an average car to be 4 m long, how many car lengths is that?
Where the speed goes
(Higher tier) To stop, the brakes apply a braking force, and a greater force produces a greater deceleration, from F = m a. The brakes are doing work against friction, and the car's kinetic energy ends up in the thermal store of the brake discs, pads and surrounding air. That is not a side effect, it is the mechanism: stopping a car means finding somewhere to put its energy. It also explains two things drivers know. Brakes get hot on a long descent, and if they get hot enough they fade, because material that is already very hot cannot absorb much more. And a wet road lengthens the stopping distance because less grip means a smaller braking force, so by W = F s the same energy needs a longer distance to remove.
Find the braking force
A car of mass 800 kg brakes with a deceleration of 5 m/s². What is the size of the braking force, in newtons (N)?
A tired driver, a wet road
A tired driver is travelling on a wet road when a hazard appears ahead. Four questions.
- Tiredness has lengthened the driver's reaction time. Why does that translate into extra DISTANCE rather than just extra time?
- The wet road gives the tyres less grip. Explain why that lengthens the braking distance.
- The driver says he leaves a fixed two-car gap at every speed. Is that sound?
- After the stop, the brake discs are hot. What does that tell you?
Which half does each change affect?
- The driver glances at a phone
- The road is icy
- The car is driven twice as fast
- The car is fitted with new tyres and freshly serviced brakes
- Thinking distance only: the reaction time is longer, and the car covers more ground before braking starts
- Braking distance only: less grip means a smaller braking force, so a longer distance to stop
- Both, but not equally: the thinking half roughly doubles while the braking half roughly quadruples
- Braking distance only, and it makes it SHORTER rather than longer
Stopping summary
Stopping distance is the thinking distance plus the _____ distance. The thinking distance depends on the driver's _____ time and grows in proportion to speed, while the braking distance depends on the road, the tyres and the mass, and grows with speed _____. The brakes stop the car by doing work against friction, transferring its kinetic energy to the _____ store.
Explain the speed limit
Exam practice. In about 50 words, explain why doubling a car's speed MORE than doubles its stopping distance. Include:
- what happens to the thinking distance, and why it changes the way it does
- what happens to the braking distance, and how that differs
- what the two together mean for the total, and for the gap a driver should leave