Stopping distance, thinking distance and braking distance
Definition
Stopping distance
Stopping distance is the total distance a vehicle travels from the moment the driver sees a hazard to the moment the vehicle stops.
- The stopping distance has two parts: the thinking distance and the braking distance.
- The thinking distance is the distance the vehicle travels during the driver’s reaction time, before the brakes are applied.
- The braking distance is the distance the vehicle travels while the braking force slows it to a stop.
- These combine so that stopping distance=thinking distance+braking distance\text{stopping distance} = \text{thinking distance} + \text{braking distance}stopping distance=thinking distance+braking distance.
- A driver cannot stop instantly: they first react to the hazard, covering the thinking distance, then the brakes apply a force that slows the vehicle, covering the braking distance.
Key Idea
The stopping distance is not only the distance travelled while braking; it also includes the distance travelled before the brakes begin to act.
How speed affects stopping distance
- For a given braking force, the greater the speed of the vehicle, the greater the stopping distance.
- A faster vehicle travels further during the driver’s reaction time, so the thinking distance is greater.
- A faster vehicle must also be slowed from a higher speed, so it travels further while braking and the braking distance is greater.
- You should be able to estimate how the distance for an emergency stop varies over a range of speeds typical for a particular vehicle.
- For example, if a car’s speed rises from 10 m/s10\ \text{m/s}10 m/s to 20 m/s20\ \text{m/s}20 m/s, the stopping distance does not stay the same; it increases, and the braking part rises especially steeply.
Example
A car has a thinking distance of 12 m12\ \text{m}12 m and a braking distance of 24 m24\ \text{m}24 m during an emergency stop. Calculate the stopping distance.
- Write the equation: stopping distance=thinking distance+braking distance\text{stopping distance} = \text{thinking distance} + \text{braking distance}stopping distance=thinking distance+braking distance.
- Substitute the values: stopping distance=12 m+24 m\text{stopping distance} = 12\ \text{m} + 24\ \text{m}stopping distance=12 m+24 m.
- The stopping distance is 36 m36\ \text{m}36 m.
Interpreting speed–stopping distance graphs
- You may be given a graph relating speed to stopping distance for a range of vehicles, with speed on the horizontal axis and stopping distance on the vertical axis.
- To read a stopping distance from such a graph:
- find the speed on the horizontal axis;
- move up to the line for the correct vehicle;
- move across to the vertical axis and read the stopping distance, usually in m\text{m}m.
- To compare vehicles at the same speed, look vertically above that speed: the vehicle with the higher point has the greater stopping distance.
- Read values carefully and include the correct units, usually m\text{m}m for distance and m/s\text{m/s}m/s for speed.
Self review
- What are the two parts that make up the stopping distance?
- Write the word equation linking stopping distance, thinking distance and braking distance.
- For a given braking force, how does increasing speed change the stopping distance, and why?
- How do you read a stopping distance from a speed–stopping distance graph?
