By how much does the braking distance increase, if you double the speed?
Show the correct answer
Correct answer
C. 4 times.
Explanation
Doubling the speed makes the braking distance four times longer - answer C. This comes from physics: the energy of motion increases with the square of the speed, and the brakes must get rid of all of it. For example, the braking distance from 50 km/h on dry asphalt is about 14 m, and from 100 km/h about 56 m.
On top of this there is the distance travelled during the reaction time, which increases in proportion to speed, so the total stopping distance at double the speed is about three times longer. That is why on a motorway the Road Traffic Act requires you to keep a distance in metres equal to at least half the speed in km/h - at 140 km/h this is 70 m.
Legal basis: Art. 19 para. 1 and para. 3a of the Road Traffic Act (Prawo o ruchu drogowym)