Section 1 of 5Components of Stopping Distance
The distance a vehicle needs to stop is made up of two parts, which respond to different factors.
Thinking distance
- Thinking distance The distance travelled during the driver's reaction time, before the brakes are applied.
Braking distance
- Braking distance The distance travelled from the moment the brakes are applied until the vehicle comes to rest.
Stopping distance
- Stopping distance Thinking distance plus braking distance, so the total stopping distance is always longer than either part on its own.
Reaction time
- Reaction time The time between seeing a hazard and applying the brakes, which varies from person to person and depends on alertness.
Speed
- Speed Thinking distance increases in proportion to speed, while braking distance increases much faster than speed.
Section 2 of 5How Stopping Distance Is Found
Stopping distance questions are answered by reading values for each part and adding them together.
1
Read the thinking distance for the given speed from a given table (or a graph).
2
Read the braking distance for the same speed, which depends on the vehicle, its brakes and the road conditions.
3
Add the thinking distance and the braking distance together to find the total stopping distance.
4
Note how changing a single factor affects only the relevant part of the total, such as reaction time changing thinking distance only.
5
Explain any change using reaction time, friction and kinetic energy where the question asks for a reason.
| Factor | Effect on thinking distance | Effect on braking distance |
|---|---|---|
| Speed | Increases it | Increases it, roughly with the square of speed |
| Tiredness or distraction | Increases it | No direct effect |
| Alcohol or drugs | Increases it | No direct effect |
| Wet or icy road | No direct effect | Increases it, because friction is reduced |
| Worn brakes or tyres | No direct effect | Increases it, because less friction is available |
Section 3 of 5Making Roads Safer
Lower speeds, good vehicles and good roads all reduce the risk of collisions.
Making Roads Safer
Speed limits
- Speed limits Lower speeds reduce both thinking and braking distances, so collisions are less severe when they happen.
Vehicle maintenance
- Vehicle maintenance Good brakes and tyres keep friction high, so vehicles stop in shorter distances.
Driver awareness
- Driver awareness Good attention and training reduce reaction time and let drivers anticipate hazards sooner.
Road design
- Road design Good road surfaces with high friction and effective drainage shorten braking distances in wet weather.
Section 4 of 5Worked Example
Question
- A car travels at 20 m/s and has a thinking distance of 12 m and a braking distance of 20 m.
- Find the stopping distance.
- If the speed doubles to 40 m/s, the thinking distance doubles and the braking distance becomes four times larger.
- Find the new stopping distance.
- Stopping distance at 20 m/s = 12 m + 20 m = 32 m.
- New thinking distance at 40 m/s = 2 × 12 m = 24 m.
- New braking distance at 40 m/s = 4 × 20 m = 80 m.
- New stopping distance = 24 m + 80 m = 104 m.
Answer The stopping distance is 32 m at 20 m/s and 104 m at 40 m/s.
Section 5 of 5Exam Checklist & Tips
5.8.2 – Key Points for the Exam
- Thinking distance The distance a vehicle travels during the driver's reaction time.
- Braking distance The distance a vehicle travels after the brakes are applied until it stops.
- Stopping distance The sum of thinking distance and braking distance.
- Reaction time The time taken for a driver to respond to a hazard by applying the brakes.
- Friction The force that opposes motion between two surfaces in contact, which slows a vehicle when the brakes are applied.
Common Mistakes
- Assuming braking distance rises in direct proportion to speed;
- at higher speeds it increases much faster than speed.
- Forgetting that reaction time is the period before the brakes are applied, so it affects thinking distance rather than braking distance.
- Stating that tiredness mainly affects braking distance; it mainly lengthens the thinking distance.
- Confusing braking distance with braking time, which are different quantities measured in different units.