Stopping distances
Stopping distance
The of a vehicle is the distance it travels from when the driver sees a hazard to when it stops:
stopping distance = thinking distance + braking distance
- how far the vehicle goes during the driver's , before the brakes go on.
- how far it goes while the brakes are slowing it down.
For a given braking force, the faster the vehicle, the longer the stopping distance.
Example: a car travels at 20 m/s and the driver's reaction time is 0.5 s.
thinking distance = speed × time = 20 × 0.5 = 10 m
Key terms
What affects each distance
are typically 0.2 s to 0.9 s. They are longer if the driver is tired, has taken drugs or alcohol, or is distracted, for example by a phone.
| Thinking distance increases with | Braking distance increases with |
|---|---|
| Higher | Higher speed |
| Tiredness | Wet or icy roads |
| Alcohol or drugs | Worn brakes or worn tyres |
| Distractions | A heavier, more loaded vehicle |
When the brakes are applied, friction between the brakes and the wheels does , transferring the car's to the store of the brakes. The brakes get hot.
Braking distance depends on the speed squared: doubling the speed makes the braking distance about four times longer.
Exam tip
Don't mix up the factors. Alcohol affects thinking distance; a wet road affects braking distance. Speed affects both.
Key terms
Measuring reaction time
The ruler drop test is a simple way to measure reaction time:
- A partner holds a metre ruler vertically, with the zero mark between your open finger and thumb.
- Without warning, they drop it. You catch it as quickly as you can.
- Read the distance the ruler fell.
- Repeat several times and find the mean.
The further the ruler falls, the slower your reaction. You can work out the time from the distance.
Example: the ruler falls 0.20 m. It falls with acceleration 9.8 m/s².
- v² − u² = 2as, so v² = 2 × 9.8 × 0.20 = 3.92, and v = 1.98 m/s
- t = Δv ÷ a = 1.98 ÷ 9.8 = 0.20 s
Use the same hand and the same person dropping the ruler each time.
Estimating braking force (Higher tier only)
The by the brakes equals the the car loses:
Example: a 1200 kg car at 20 m/s stops in a braking distance of 40 m. Estimate the braking force.
- Kinetic energy = ½mv² = 0.5 × 1200 × 20² = 240 000 J
- Braking force = 240 000 ÷ 40 = 6000 N
The faster a vehicle goes, the bigger the braking force needed to stop it in a given distance. A very large deceleration can make the brakes overheat, or make the driver lose control as the wheels skid.