A driver's reaction time is not fixed. Several factors make it longer:
- Drugs: alcohol and many other drugs slow the response of the brain, and some medicines cause drowsiness. They also affect a driver's judgement.
- Tiredness: a tired driver reacts more slowly.
- Distractions: using a mobile phone, talking to passengers or loud music take attention away from the road.
A longer reaction time means the car travels further before the brakes are applied, so the thinking distance is greater. Thinking distance = speed × reaction time. For example, at 20 m/s a reaction time of 0.2 s gives a thinking distance of 4 m, but a reaction time of 0.6 s gives 12 m.
The braking distance is unchanged by reaction time, but the stopping distance still increases because the thinking distance is part of it. This makes a collision more likely.