5.8.4Forces
Factors Affecting Thinking Distance and Reaction Time
5 min read 5 parts Lesson 60 of 67 Unit quiz
Section 1 of 5Factors That Change Reaction Time
Reaction time is the time a driver takes to respond to a hazard, and it can be made longer by several conditions.
Tiredness
- Tiredness A tired driver reacts more slowly, which lengthens the time before the brakes are applied.
Alcohol
- Alcohol Alcohol slows reactions and impairs judgement, so the driver takes longer to respond to hazards.
Drugs
- Drugs Many legal and illegal drugs slow reaction time, sometimes significantly, and affect the driver's concentration.
Distractions
- Distractions Using a mobile phone or being distracted takes attention away from the road and delays the response.
Section 2 of 5Effects on Thinking Distance
Thinking distance is the distance travelled during the reaction time, so anything that changes reaction time or speed changes it.
Effects on Thinking Distance
Speed of the car
- Speed of the car Thinking distance increases with speed, because the car covers more distance in the same reaction time.
Reaction time
- Reaction time A longer reaction time means the car travels further before the driver starts to brake.
Driver condition
- Driver condition Tiredness, alcohol and drugs all increase reaction time and so increase the thinking distance.
| Factor | Effect on reaction time | Effect on thinking distance |
|---|---|---|
| Higher speed of the car | No direct effect | Increases, because more distance is covered in the same reaction time |
| Tiredness | Increases | Increases |
| Alcohol | Increases | Increases |
| Drugs | Increases | Increases |
Section 3 of 5How a Hazard Is Handled
Stopping a car takes place in clear stages, and the reaction time is only the first of them.
1
The driver sees the hazard, and the brain processes the information before deciding what to do.
2
The driver's foot moves from the accelerator to the brake, and this delay is the reaction time.
3
During the reaction time the car travels the thinking distance at its current speed.
4
Once the brakes are applied, the car slows down, and the braking distance begins to add to the total.
Section 4 of 5Worked Example
Question
- A car travels at 20 m/s and the driver's reaction time is 0.5 s.
- Calculate the thinking distance.
- The driver then becomes tired and the reaction time doubles to 1.0 s.
- Calculate the new thinking distance.
- Thinking distance = speed × reaction time.
- Original thinking distance = 20 × 0.5 = 10 m.
- New thinking distance = 20 × 1.0 = 20 m.
- Doubling the reaction time doubles the thinking distance, because the speed has stayed the same.
Answer 10 m originally, rising to 20 m when the reaction time doubles.
Section 5 of 5Exam Checklist & Tips
5.8.4 – Key Points for the Exam
- Thinking distance The distance a vehicle travels during the driver's reaction time.
- Reaction time The time between a driver seeing a hazard and applying the brakes.
- Braking distance The distance a vehicle travels once the brakes have been applied.
- Stopping distance The sum of the thinking distance and the braking distance.
- Hazard Anything on the road that could cause a collision, such as an obstacle or a pedestrian.
Common Mistakes
- Saying braking distance is the same as thinking distance, when braking begins only after the reaction time has finished.
- Stating that thinking distance depends on the car's brakes, when it depends on speed and reaction time.
- Using milliseconds in the thinking distance equation without converting reaction time into seconds.
- Claiming that speed does not affect thinking distance, when a higher speed covers more distance in the same reaction time.