By approximately how much does the braking distance of a bus increase if the speed doubles?
Show the correct answer
Correct answer
C. Four times.
Explanation
The question is about how the braking distance of a bus changes when the speed doubles. It becomes approximately four times longer - answer C.
The Road Traffic Act requires you to drive at a speed that ensures control over the vehicle and to keep the following distance needed to avoid a collision if the vehicle in front brakes. The braking distance grows with the square of the speed, because this is how the energy of motion grows, and the brakes must turn this energy into heat: twice as fast means a four times longer distance, and three times as fast - nine times longer. Answer A assumes a simple proportion, and answer B understates the effect. In a bus with passengers this is particularly important, because you cannot brake sharply without the risk that standing passengers will fall over. On top of the braking distance there is also the distance covered during your reaction time.
Legal basis: Art. 19 para. 1 and para. 2 point 3 of the Road Traffic Act (Prawo o ruchu drogowym)