A car is travelling at constant speed along a motorway when the driver sees a warning sign and must stop the vehicle. Explain all the factors that affect the total stopping distance of the car.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Stopping distance is made up of thinking distance and braking distance.
Model answer (5 marks)
Stopping distance is the sum of thinking distance and braking distance.
Thinking distance depends on the driver’s reaction time. Reaction time is longer if the driver is tired, under the influence of drugs or alcohol, or distracted. The faster the vehicle is moving, the further it travels during that reaction time, so thinking distance increases with speed.
Braking distance is influenced by the road and weather conditions. Wet or icy roads reduce the friction between tyre and road, so the car takes longer to stop. The condition of the vehicle also matters: worn tyres or brakes give less grip and increase braking distance.
Thinking distance depends on the driver’s reaction time. Reaction time is longer if the driver is tired, under the influence of drugs or alcohol, or distracted. The faster the vehicle is moving, the further it travels during that reaction time, so thinking distance increases with speed.
Braking distance is influenced by the road and weather conditions. Wet or icy roads reduce the friction between tyre and road, so the car takes longer to stop. The condition of the vehicle also matters: worn tyres or brakes give less grip and increase braking distance.
Examiner tips
- State that stopping distance = thinking distance + braking distance; explain each component. Mention reaction time factors and speed effect on thinking distance. Explain how road conditions and vehicle condition affect braking distance. Use correct terminology (friction, tyre grip).
Common mistakes
- Forgetting to include both thinking and braking distance. Failing to link speed to increased reaction distance. Not mentioning tyre or brake condition as a factor.
Mark scheme (5 marks)
- Stopping distance is the sum of thinking distance and braking distance
- Thinking distance is affected by the driver's reaction time / factors that increase reaction time (e.g. tiredness, drugs, alcohol)
- Greater speed increases thinking distance (the vehicle travels further during the reaction time)
- Braking distance is affected by adverse road or weather conditions (e.g. wet or icy roads reduce grip / friction)
- Braking distance is affected by the condition of the vehicle (e.g. worn tyres or brakes)
Key terms in this question
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