Braking distance
Enter the speed and reaction time: calculate the reaction, braking and total stopping distance.
How it works
Stopping distance is the sum of the distance covered during the reaction time (before braking) and the actual braking distance, which grows with the square of speed.
Note
A deceleration of ~7 m/s² is assumed (dry asphalt, good tires). On wet or snow the distance increases a lot. Doubling the speed quadruples the braking distance!
The two pieces a stop is made of
Stopping distance is the sum of two different stretches: the one covered during reaction time, in which the car is still at full speed, and the braking itself. The first grows in proportion to speed, the second with its square.
That is why the numbers look exaggerated at high speed: at 50 km/h you need some fifteen metres to react and as many again to stop, at 100 km/h you need 28 to react and more than 55 to stop. Double the speed and the distance does not double, it nearly quadruples.
The surface changes everything
The standard sum assumes dry asphalt and tyres in good condition, meaning a deceleration of around 7 m/s². In the wet it drops to 4-5, on snow to 2, on ice below 1: the same 55 metre braking distance becomes 90 in the wet and over 200 on ice.
Worn tyres count as much as rain too: below 3 mm of tread the ability to clear water collapses, and on a wet surface the distance stretches by tens of metres. The legal limit is 1.6 mm, but that is the minimum for being on the road, not the depth at which a tyre brakes well.
What ABS does and does not do
ABS does not shorten braking distance in a straight line, or shortens it only slightly: what it does is stop the wheels locking, so the car stays steerable. In an emergency stop you can swerve round the obstacle, and that is the real life-saver.
The right technique with ABS is to press hard and stay pressed, without pumping and without easing off when the pedal shudders: the shudder is the system working. Drivers who learned before ABS tend to release, and that is exactly the moment they lose the braking.
Nearby tools
How far to stay behind the car ahead is answered by Safe following distance. The tyres doing the work are chosen with Tyre sizes and decoded with Read the tyre code.