Compared with 20 mph, approximately how much braking distance is required at 40 mph under the same conditions?
Correct answer: B – Four times as much
The New Hampshire CDL manual explains that doubling speed from 20 mph to 40 mph makes braking distance approximately four times longer. This comparison concerns braking distance, which begins once the brakes are applied. Total stopping distance also includes perception and reaction distance.
Source: New Hampshire Commercial Driver’s License Manual — § 2.6.1, “Stopping Distance”

