S4.8.8 For the adult
Stopping distances
A child steps into the road. Why can a car not stop the moment the driver sees her?
What this chapter teaches
Stopping distance is thinking distance plus braking distance. Thinking distance is speed times reaction time. Braking distance depends on the speed, the grip, the brakes and the load, and it grows with the square of the speed.
Three questions to ask
There is no one right wording. Listen for the idea.
- A car travels at 20 m/s and the driver’s reaction time is 0.7 s. How far does it go before the brakes start to work?
- Why does doubling the speed more than double the stopping distance?
- Which of tiredness, a wet road, worn tyres and a phone call change the thinking distance, and which change the braking distance?
After the chapter, the learner can
- Explain what affects thinking distance and braking distance
- Use a measured reaction time to work out thinking distance
What they copy into their notebook
Stopping distance = thinking distance + braking distance. Thinking distance = speed × reaction time. Braking distance grows with the square of the speed. Wet roads and heavy loads make it longer.