By what factor does stopping distance increase when speed is doubled?

• A. By 2 times
• B. By 4 times
• C. By 8 times
• D. By 1/2

Answer: (B)

Stopping distance grows in two parts: thinking distance and braking distance. The braking portion, which is the distance you slide to a stop once you start braking, increases with the square of speed. That comes from the basic braking equation d = v^2/(2a) for a constant deceleration a. So when you double your speed, the braking distance becomes four times as long. The thinking distance, which is speed times your reaction time, would double, but at typical driving speeds the braking distance dominates the total stopping distance. Therefore doubling speed leads to about a fourfold increase in total stopping distance.