According to the CDL manual, if a driver doubles speed from 20 mph to 40 mph, how much greater does the impact and braking distance become?

49 CFR 383.111(a)(9); AAMVA CDL Manual (nhcdm.pdf) Section 2.6.1, p. 2-14, 'The Effect of Speed on Stopping Distance'

This is one of the most specific, testable facts in the speed-management section of the exam. It explains why even modest speeding has an outsized effect on crash severity and stopping ability, speed doesn't just add distance, it multiplies it, which is why high-speed crashes involving large commercial vehicles are disproportionately severe.