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Stopping distances
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A car cannot stop instantly. The total stopping distance has two parts.
Two parts
Stopping distance = thinking distance + braking distance
Thinking distance: travelled during the driver's reaction time.
Braking distance: travelled while the brakes slow the car.
Thinking distance: travelled during the driver's reaction time.
Braking distance: travelled while the brakes slow the car.
What affects each part
Thinking distance: speed, tiredness, alcohol, drugs, distractions such as phones.
Braking distance: speed, wet or icy roads, worn tyres, worn brakes, the mass of the car.
Braking distance: speed, wet or icy roads, worn tyres, worn brakes, the mass of the car.
Reaction time
A typical reaction time is 0.2 to 0.9 s.
Thinking distance = speed × reaction time.
Doubling the speed doubles the thinking distance but roughly quadruples the braking distance (kinetic energy depends on v2).
Thinking distance = speed × reaction time.
Doubling the speed doubles the thinking distance but roughly quadruples the braking distance (kinetic energy depends on v2).
A driver travelling at 20 m/s has a reaction time of 0.7 s. Find the thinking distance.
- Thinking distance = 20 × 0.7
Answer: 14 m
Stopping distance = thinking distance + braking distance. Tiredness, alcohol and distraction increase thinking distance; wet roads and worn tyres or brakes increase braking distance.
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