Commercial Driver License (CDL) Class B Practice Test

What effect does speed have on stopping distance?

1. Higher speed leads to a shorter stopping distance

2. Speed has no effect on stopping distance

3. Doubling speed quadruples stopping distance

4. Speed only matters on flat roads

The correct answer is: Doubling speed quadruples stopping distance

The principle that doubling speed results in quadrupling stopping distance is rooted in the physics of motion. The stopping distance is made up of two components: the perception/reaction distance and the braking distance. When speed increases, the time needed for a driver to perceive a hazard and react appropriately also increases. However, the braking distance is affected more significantly by speed. Specifically, braking distance increases with the square of the speed. This means that if a vehicle is traveling at twice the speed, the kinetic energy that needs to be dissipated during braking increases fourfold, which in turn requires a greater distance to come to a complete stop. This understanding highlights the importance of maintaining safe speeds, especially in conditions where stopping distance may be critical, such as in heavy traffic or approaching intersections. In contrast, the other choices do not accurately reflect the relationship between speed and stopping distance.