5.6.3c Factors affecting braking distance
Braking distance
Definition
Braking distance
Braking distance is the distance a vehicle travels from the moment the brakes are applied to the moment it comes to a complete stop.
- The braking distance is one part of the stopping distance, where stopping distance=thinking distance+braking distance\text{stopping distance} = \text{thinking distance} + \text{braking distance}stopping distance=thinking distance+braking distance.
- Braking relies on the friction between the tyres and the road; this friction provides the braking force, which does work to slow the vehicle down.
- A longer braking distance means the vehicle may collide with a hazard before it stops, so drivers must leave a safe gap and adjust their speed to the conditions.
Key Idea
Anything that reduces the friction between the tyres and the road, or reduces the braking force, increases the braking distance.
Road and weather conditions
- Braking depends on the friction between the vehicle’s tyres and the road, which provides the force that slows the vehicle.
- On a wet road, water reduces the grip between the tyres and the road, lowering the friction, so the vehicle decelerates less and travels further before stopping.
- On an icy road the friction is much lower still, so the braking distance can be much longer.
- In poor conditions a driver should reduce their speed and leave a larger gap from the vehicle in front, because more distance is needed to stop safely.
Condition of the vehicle
- The condition of the vehicle’s tyres and brakes also affects its braking distance.
- Tyres need enough tread to grip the road and to clear water from the contact patch; worn tyres give less grip, especially in the wet, reducing friction and increasing the braking distance.
- Brakes must be in good condition to produce a large enough braking force; worn brakes produce a smaller force, giving a smaller deceleration and a longer braking distance.
- Poorly maintained brakes or tyres therefore increase the risk of a collision, so vehicles should be well maintained and drivers should reduce speed when conditions are poor.
Exam technique
When explaining why a factor increases braking distance, give the full chain: the factor reduces the friction or braking force, so the deceleration is smaller, so the vehicle travels further before stopping. Then state the safety implication, such as reducing speed or leaving a larger gap.
The effect of speed
- The greater the speed, the further the vehicle travels before stopping; braking distance rises steeply, so doubling the speed makes the braking distance more than double under the same conditions.
- For an alert driver, a dry road and a well-maintained vehicle, typical overall stopping distances are approximately:
- 12 m12\ \text{m}12 m at 20 mph20\ \text{mph}20 mph, and 23 m23\ \text{m}23 m at 30 mph30\ \text{mph}30 mph;
- 36 m36\ \text{m}36 m at 40 mph40\ \text{mph}40 mph, and 53 m53\ \text{m}53 m at 50 mph50\ \text{mph}50 mph;
- 73 m73\ \text{m}73 m at 60 mph60\ \text{mph}60 mph, and 96 m96\ \text{m}96 m at 70 mph70\ \text{mph}70 mph.
- These are estimates rather than guaranteed distances: wet or icy roads and worn brakes or tyres can make the actual distance much greater.
Example
A car’s typical stopping distance is about 23 m23\ \text{m}23 m at 30 mph30\ \text{mph}30 mph and 73 m73\ \text{m}73 m at 60 mph60\ \text{mph}60 mph. Estimate the factor by which the stopping distance increases when the speed doubles.
- Write the relationship: factor=new distanceoriginal distance\text{factor} = \frac{\text{new distance}}{\text{original distance}}factor=original distancenew distance.
- Substitute the values: factor=73 m23 m\text{factor} = \frac{73\ \text{m}}{23\ \text{m}}factor=23 m73 m.
- The stopping distance increases by a factor of about 3.23.23.2, so doubling the speed much more than doubles the stopping distance.
Safety implications
- Because braking distance rises steeply with speed and depends on the road and the vehicle, drivers must match their speed to the conditions.
- On wet or icy roads, or with worn tyres or brakes, a driver should reduce speed and leave a larger gap so there is enough distance to stop in an emergency.
- Do not assume that doubling the speed only doubles the braking distance; it increases by much more, and braking distance is measured only from when the brakes are applied.
Self review
- What is the difference between braking distance and stopping distance?
- Why do wet or icy roads increase the braking distance?
- How do worn tyres and worn brakes each increase the braking distance?
- What happens to the braking distance when the speed of a vehicle doubles?
