By approximately how much does the braking distance of a truck increase if the speed doubles?
Show the correct answer
Correct answer
C. Four times.
Explanation
In the photo, you can see the wheels of the drive axle of a lorry and a spare wheel - the grip of the tyres determines how quickly you can stop the combination. When the speed is doubled, the braking distance of a lorry increases approximately four times - answer C.
The Road Traffic Act requires you to drive at a speed that ensures control of the vehicle, taking into account the traffic conditions, the condition of the road and the vehicle's load. The braking distance depends on the energy of motion, and this grows with the square of the speed: at twice the speed, the brakes must get rid of four times as much energy, so they need a distance four times as long. In a lorry this is especially important, because its mass also plays a part: a 40-tonne combination travelling at 80 km/h needs from a dozen or so to several dozen metres more to stop than a passenger car, and drum brakes heat up during long braking and lose effectiveness. That is why you estimate the following distance in seconds, and on a motorway and an expressway the rules give it in metres - at least half of the number that shows the speed in km/h.
Legal basis: Art. 19 para. 1, para. 2 point 3 and para. 3a of the Road Traffic Act (Prawo o ruchu drogowym)