Stopping Distances

The equation

stopping distance = thinking distance + braking distance

Thinking distance

Distance travelled during the driver's reaction time (before braking).

• Increased by: tiredness, alcohol, drugs, distractions (phone), higher speed.

Braking distance

Distance travelled while braking to a stop.

• Increased by: wet/icy roads, worn tyres, worn brakes, higher speed, greater mass.

Effect of speed

• Thinking distance is proportional to speed.
• Braking distance is proportional to speed² — so doubling speed quadruples braking distance.

Energy in braking

Braking transfers the car's kinetic energy to thermal energy in the brakes (work done by friction). Large decelerations mean the brakes can overheat and forces on passengers can be dangerous.

Exam tip

Separate the two factors clearly: thinking distance ↔ the driver/reaction; braking distance ↔ the car/road conditions. Braking distance grows with speed squared.