You need about how many times as much stopping distance at 40 mph as at 20 mph?

You need about how many times as much stopping distance at 40 mph as at 20 mph?

• A. 2 times
• B. 3 times
• C. 4 times
• D. 5 times

Answer: (C)

The main idea is that stopping distance grows roughly with the square of your speed. The braking distance is tied to how much kinetic energy you have to dissipate, which is proportional to v^2. If you double your speed from 20 mph to 40 mph, the energy to remove and the distance over which you must apply brakes increase by about a factor of four, so stopping distance becomes about four times as long. The total stopping distance also includes the perception/reaction distance (which grows with speed), but the braking part dominates at higher speeds, making the fourfold increase the best practical rule of thumb.

The main idea is that stopping distance grows roughly with the square of your speed. The braking distance is tied to how much kinetic energy you have to dissipate, which is proportional to v^2. If you double your speed from 20 mph to 40 mph, the energy to remove and the distance over which you must apply brakes increase by about a factor of four, so stopping distance becomes about four times as long. The total stopping distance also includes the perception/reaction distance (which grows with speed), but the braking part dominates at higher speeds, making the fourfold increase the best practical rule of thumb.