How many feet must you have to stop while driving 55 miles per hour?

Answer 1

The UK highway code uses the formula:

distance = thinking_distance + braking_distance

= speed + speed2 ÷ 20

with speed in mph and distance in feet. This is for a normal, unaffected, person in a vehicle with good brakes and tyres, and dry conditions.

So for 55 mph, the stopping distance would be:

distance = 55 + 552 ÷ 20

= 55 + 151.25

= 206.25 ft

≈ 63m

If the driver has different than "normal" reaction times and the road is slippery (due to rain, snow, dust(!), etc), then the stopping distance will vary, usually longer, the Highway Code suggesting allow double at least.

However, as far as I know the Highway Code has not been updated for the vast improvements in braking, etc since it was first published; also, the same stopping distances are considered for a (less than) 1 tonne family car and fully laden 23 tonne coach.

Fun fact: the stopping distance (given above) for 70 mph (the legal limit for a car on the UK motorways) is 315 ft or 96m; this can easily be visualised as the marker posts to the nearest emergency phone are 100m apart - approximately the stopping distance in an emergency!