The Stopping distance of a car is the distance it takes to come to a standstill. It is therefore dependent on the current speed. Knowing stopping distances is part of the theory test section of the British Driving test.
The Stopping distance comprises of two parts. The thinking distance (ie how long it takes from when your eyes see a stationary object in front of you until you press the brake pedal), and the braking distance (ie the time it takes for the car to actually come to a standstill once the brakes are applied).
The standard distances are given as follows in the current Highway Code.
- At 20 mph, thinking distance is 6 metres and braking distance is 6 metres - total of 12 metres / 40 feet, or about 3 car lengths.
- At 30 mph, thinking distance is 9 metres and braking distance is 14 metres - total of 23 metres / 75 feet, or about 6 car lengths.
- At 40 mph, thinking distance is 12 metres and braking distance is 24 metres - total of 36 metres / 120 feet, or about 9 car lengths.
- At 50 mph, thinking distance is 15 metres and braking distance is 38 metres - total of 53 metres / 175 feet, or about 13 car lengths.
- At 60 mph, thinking distance is 18 metres and braking distance is 55 metres - total of 73 metres / 240 feet, or about 18 car lengths.
- At 70 mph, thinking distance is 21 metres and braking distance is 75 metres - total of 96 metres / 315 feet, or about 24 car lengths.
These distances are based around a standard car with standard brakes, in good conditions. If conditions are bad, it's going to take longer to stop! Braking too sharply in wet / icy conditions can cause your wheels to lock (although anti-lock brakes can help avoid this).
Officially you have to memorise these distances as guides for the driving test. However, there is a formula that can be used to calculate the stopping distance (in feet) for any speed (in mph).
- The thinking distance in feet is the speed in mph - at 30 mph, thinking distance is 30 feet.
- The braking distance in feet is the speed in mph squared, divided by 20 - at 30 mph, the braking distance is 30 * 30 / 20 = 45 feet.
- So at 30 mph, total stopping distance = 30 + 45 = 75 feet.
To put it another way, total stopping distance in feet is v2 / 20 + v where v is your speed in mph.
gbulmer says from memory, the 'standard' car that the stopping distances is a Ford Anglia, drum brakes only on front! They have been unchanged since 1965... I've no idea if this is the case but it wouldn't surprise me - and therefore modern cars would stop much quicker. But it never does any harm to leave a bigger gap!