Washington State Department of Licensing Knowledge Test

At 60 mph, how many times farther does it take to stop compared to 30 mph?

1. 2 times

2. 3 times

3. 4 times

4. 5 times

The correct answer is: 3 times

To understand why at 60 mph it takes four times farther to stop compared to 30 mph, one must consider the physics involved in stopping a vehicle. The stopping distance of a vehicle is determined by both the reaction time and the braking distance, which is influenced by the speed of the vehicle. The formula for braking distance can be expressed as follows: stopping distance is proportional to the square of the speed. This means if you increase the speed, the stopping distance increases by the square of the change in speed. When you’re traveling at 30 mph and want to consider what happens at 60 mph, you can set it up like this: - At 30 mph, if we denote the stopping distance as X, at 60 mph the stopping distance would be (60/30)² times X, which simplifies to 2² times X, equating to 4X. Therefore, at 60 mph, it takes four times farther to stop compared to 30 mph. This squared relationship illustrates how speed dramatically impacts stopping distance, emphasizing the importance of driving at safe speeds and maintaining an appropriate distance from other vehicles.