SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/cdl-doubling-speed-stopping

If you double your speed from 20 to 40 mph, what happens to braking distance?

Braking distance becomes 4 times longer.

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  1. It doubles
  2. It triples
  3. It quadruples ✔

Why: Braking distance grows much faster than speed. Doubling speed from 20 to 40 mph makes it 4 times longer, and tripling to 60 mph makes it 9 times longer.

Huh, didn't know that: The impact, or striking power, of the vehicle is also 4 times greater at double the speed.

The dad joke
Why did the trucker fail math? He thought doubling speed only doubled the trouble.
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References

Answer: It quadruples. Braking distance grows much faster than speed. Doubling speed from 20 to 40 mph makes it 4 times longer, and tripling to 60 mph makes it 9 times longer.

  1. California Commercial Driver Handbook (AAMVA model manual) www.dmv.ca.gov/portal/file/california-commercial-driver-handbook-pdf
    When you double your speed from 20 to 40 mph, the impact is 4 times greater. The braking distance is also 4 times longer.
    Checked 2026-10-10.

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