If speed increases from 20 mph to 40 mph, approximately how does braking distance change under comparable conditions?
Correct answer: B. – It becomes about four times as long
Doubling speed from 20 mph to 40 mph makes braking distance approximately four times longer under comparable conditions. Total stopping distance also includes perception and reaction distances, not merely the distance traveled after the brakes are applied. Higher speed additionally increases impact force, making crashes more severe and leaving less room to correct driving errors.
Source: Georgia Commercial Driver's Study Guide — § 2.6.1, “The Effect of Speed on Stopping Distance”

