How does doubling of the lorry speed influence the length of the braking distance?
Show the correct answer
Correct answer
B. It increases the braking distance by four times.
Explanation
The question is about the effect of doubling the speed of a lorry on its braking distance. The braking distance becomes four times longer - answer B.
The Road Traffic Act requires you to drive at a speed that keeps the vehicle under control and to keep a following distance that lets you avoid a collision if the vehicle in front brakes. The braking distance grows with the square of the speed, because that is how much the energy of motion grows, and the brakes must turn this energy into heat: twice as fast means a braking distance four times longer, and three times as fast - nine times longer. That is why answer A underestimates the effect, and answer C goes against physics. In a lorry this relationship matters even more, because with a large mass the brakes heat up faster. Remember also that the stopping distance is the braking distance plus the distance travelled during the driver's reaction time.
Legal basis: Art. 19 para. 1 and para. 2 point 3 of the Road Traffic Act (Prawo o ruchu drogowym)