How long does it take to stop a car going 20 mph?

Answer 1

That depends upon the braking ability of the car, the condition of the tyres and road surface.

Also the alertness of the driver will have some effect.

If you are referring to the emergency stopping distance as suggested by the UK Government in The Highway Code publication (which has not been updated for modern cars) for a dry road and a car with good brakes and tyres, then the stopping distance is 20ft for the thinking distance plus an actual stopping distance of 20ft making a total of 40ft.

The table of stopping distances is given by:

d = speed + speed2 ÷ 20

where the distance d is in feet and speed is in mph.

At 70mph this equates to 315 feet ~= 96m which is almost 100 metres - the distance between marker posts along the side of a motorway!