Stopping Distance

Stopping distance is the total distance a vehicle travels from the moment the driver sees a hazard to the moment the vehicle comes to a complete stop.

The formula

stopping distance = thinking distance + braking distance

Thinking distance: the distance the vehicle travels during the driver’s reaction time (the time between seeing the hazard and pressing the brake pedal). During this time the vehicle travels at constant speed.

Braking distance: the distance the vehicle travels from the moment the brakes are applied until the vehicle stops. During this time the vehicle decelerates.

Factors that increase thinking distance

• Higher speed (the vehicle covers more distance in the same reaction time)
• Tiredness
• Alcohol or drugs
• Distractions (using a phone)
• Illness

All of these factors increase reaction time or the distance covered during the reaction time.

Factors that increase braking distance

• Higher speed (the vehicle has more kinetic energy to dissipate)
• Wet or icy road (less friction between tyres and road)
• Worn tyres or worn brake pads (less friction)
• Greater mass of vehicle (more kinetic energy for the same speed)
• Downhill slope (component of gravity acts against braking force)

Speed and braking distance

When speed doubles, kinetic energy quadruples (KE = 0.5mv squared). The braking force stays roughly the same, so the braking distance also roughly quadruples. This is why speeding is particularly dangerous: a small increase in speed produces a much larger increase in stopping distance.

Why this matters

At 30 mph, the typical stopping distance is about 23 m. At 60 mph, it increases to about 73 m. This is why speed limits are lower in residential areas and near schools.