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SESK: Stuff Everyone Should Know · sesk.info/cdl-following-distance-formula

What is the basic CDL following distance rule for speeds under 40 mph?

Use one second per 10 feet of vehicle length.

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  1. One second per 10 feet of vehicle length ✔
  2. One second per 20 feet of vehicle length
  3. Always two seconds

Why: Longer vehicles need more following distance. At speeds above 40 mph, add one extra second.

Huh, didn't know that: A 40-foot truck needs at least 4 seconds of following distance under 40 mph.

The dad joke
Why did the 40-foot truck follow so far behind? It needed space for its long personality.
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References

Answer: One second per 10 feet of vehicle length. Longer vehicles need more following distance. At speeds above 40 mph, add one extra second.

  1. California Commercial Driver Handbook (AAMVA model manual) www.dmv.ca.gov/portal/file/california-commercial-driver-handbook-pdf
    you need at least 1 second for each 10 feet of vehicle length at speeds below 40 mph.
    Checked 2026-10-10.

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