A driver is travelling along a motorway at a constant velocity when they suddenly see an obstacle on the road ahead. Explain why the total stopping distance of the vehicle depends on both the driver and the condition of the vehicle and road.

The total stopping distance is the sum of the thinking distance and the braking distance.
1. Thinking distance is the distance travelled while the driver reacts to the obstacle. It depends on the driver’s reaction time.
2. Reaction time can be lengthened by factors such as tiredness, drugs or alcohol, so a driver who is fatigued or impaired will have a longer thinking distance.
3. Braking distance is the distance travelled after the brakes are applied, which depends on the braking force and the vehicle’s speed.
4. The braking force is affected by the condition of the vehicle (e.g. brake wear, tyre condition) and by the road surface. Wet, icy or uneven roads reduce friction and therefore increase the braking distance.
5. Consequently, both the driver’s state and the vehicle/road conditions influence the total stopping distance.