As you are driving along, some part of your mind is probably constantly estimating how much space you would need to brake if the car in front of you suddenly stopped. All too often, however, drivers don't realize the difference between stopping time and stopping distance. Let's figure it out.

Stopping Time

First, let's assume the road is a uniform surface. Even though pumping your brakes is a better idea, let's also assume the stopping car is skidding to a halt, so its speed decreases uniformly. Now supposejust as a guessa car going thirty takes four seconds to skid to a halt.

How long will it take at sixty?

Did you say eight seconds? Correct. As you would expect, twice the speed equals twice the stopping time. Now here's the catch. Say that same car going thirty required a hundred feet of skidding distance before stopping.

How much distance does it need at sixty?

Did you say two hundred feet? It's sensible to guess that twice the speed equals twice the stopping distance as well. But it's wrong.

Stopping Distance

Stopping distance is the average speed of the car times the average stopping time. Since both these quantities have doubled, the stopping distance is four times as great as before, not just twice as great. The car moving at sixty requires four hundred feet of skidding room.

Realizing that stopping distance goes up twice as fast as stopping time might remind you to leave more space in front of your car than you think you need. And maybe save you from a close encounter of the most unpleasant kind.