Stopping distance

1. When a driver sees a hazard and has to stop in an emergency, the vehicle keeps moving for some distance before it comes to rest.
2. The stopping distance is the total distance travelled from the moment the driver sees the hazard to the moment the vehicle stops.
3. The stopping distance has two parts, $stopping\; distance=thinking\; distance+braking\; distance$.
4. The thinking distance is covered during the driver's reaction time, while the vehicle is still moving at its original speed.
5. The braking distance is covered after the brakes are applied, while the vehicle is slowing down.
6. The two parts are affected by different factors, so each factor must be linked to the part of the stopping distance that it changes.
7. Speed is the one factor that increases both parts.

Thinking distance

1. During the reaction time the vehicle does not slow down, so $thinking\; distance=speed\times reaction\; time$.
2. A longer reaction time gives a longer thinking distance, because the vehicle travels for longer before the brakes are applied.
3. A higher speed gives a longer thinking distance, because the vehicle covers more metres in each second of the reaction time.
4. Tiredness slows the way the brain processes information, so the driver takes longer to notice the hazard and respond.
5. Alcohol and many drugs, including some medicines, slow the nervous system and so increase the reaction time.
6. Distractions, such as using a phone, adjusting a sat nav or talking to passengers, mean the driver notices the hazard later.
7. These factors affect the driver, not the vehicle, so they change the thinking distance but not the braking distance.

Braking distance

1. When the driver presses the brake pedal, brake pads are pushed against discs that turn with the wheels, and friction between them slows the wheels.
2. The tyres grip the road, so friction between the tyres and the road provides the braking force that slows the whole vehicle.
3. The braking force does work on the vehicle, transferring energy from its kinetic energy store to the thermal energy store of the brakes and the surroundings.
4. The vehicle stops when all of its kinetic energy has been transferred, and the work done is $braking\; force\times braking\; distance$.
5. So a vehicle with more kinetic energy, or a smaller braking force, needs a longer braking distance to stop.

Factors affecting braking distance

1. Speed: a faster vehicle has much more kinetic energy, so the brakes must do more work and the braking distance increases.
2. Mass: a heavily loaded car, van or lorry has more kinetic energy at the same speed, so it travels further before the braking force stops it.
3. Brakes: worn brake pads, or brakes that have overheated, produce a smaller frictional force, so the vehicle slows less quickly.
4. Tyres: worn tyres with shallow tread grip the road less, especially in the wet, because the tread is needed to channel water away from under the tyre.
   1. In the UK the tread on a car tyre must be at least $1.6mm$ deep.
5. Road surface: water, ice, snow, loose gravel, oil and wet leaves all reduce the friction between the tyres and the road.
   1. Braking distances are at least doubled on wet roads and can be up to ten times longer on ice.
6. Slope: on a downhill slope part of the vehicle's weight acts down the slope, against the braking force, so the vehicle takes longer to stop.

Road safety

1. A driver must always be able to stop within the distance they can see to be clear, so anything that increases the stopping distance increases the risk of a collision.
2. Speed limits are lower in towns, near schools and on busy roads, because a lower speed shortens both the thinking distance and the braking distance.
3. Drivers are advised to leave a gap of at least two seconds behind the vehicle in front, and to double it on wet roads.
4. A time gap works at every speed, because a faster vehicle covers more metres in the same two seconds.
5. Drivers should slow down and leave a larger gap in rain, fog, ice and snow, and when carrying a heavy load.
6. Fog, heavy rain and darkness also mean that a hazard is seen later, which leaves less clear road in which to stop.
7. Laws against drink driving, drug driving and using a hand-held phone while driving exist because each of these lengthens the thinking distance.
8. Regular checks of brakes and tyres, including the yearly MOT test for older cars, keep braking distances short.
9. Some roads near junctions and roundabouts have special high-friction surfaces, which increase the grip of the tyres and shorten braking distances.