Road rules · CDL

Low-Clearance Underpasses

A commercial driver is required to know the vehicle's exact height, loaded and empty, and to believe posted clearances over habit and navigation apps. A posted low clearance is a hard limit: if the number is not comfortably above your height, you do not enter.

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High aerial view of a tall white box truck stopped on a one-way single-lane access road before a low underpass and striped clearance bar
The rule

How this situation is handled

The repaired plan is always the same: stop before the structure, not under it, and find another route. Bridge strikes are among the most preventable serious truck crashes, and they usually begin with a car-oriented GPS route and end with a trailer roof peeled open. Heights can also change with resurfaced pavement or snow pack, so a remembered clearance is not a measured one.

The mistake it catches

The tested mistake is trusting a navigation app or a familiar route over the posted number, or trying to creep under to check. By the time the trailer touches, the damage and the closure have already happened.

On the test

Where this scenario is tested

This situation appears in 8 questions across 8 of our state banks, on CDL practice tests. Start with your state:

Try it

A real practice question about this rule

Sample 1 · Speed Space And Road Conditions

An empty box truck approaches a low structure whose posted clearance is close to the truck's measured height. Which factor deserves special attention?

High aerial view of a tall white box truck stopped on a one-way single-lane access road before a low underpass and striped clearance bar
  1. Tire pressure has no relationship to vehicle height
  2. The empty suspension may make the vehicle ride higher than when loadedCorrect
  3. The empty cargo area automatically lowers the roof
  4. Posted clearance can be ignored when traffic is light

Why: A commercial driver's clearance check must consider posted heights, pavement changes, cargo height, and the possibility that an empty vehicle may ride higher than a loaded one. Therefore, the proposed supported response is "The empty suspension may make the vehicle ride higher than when loaded".

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Questions

Questions about low-clearance underpasses

What must a CDL driver know before any route?

The vehicle's actual height, loaded as it stands today. Every clearance decision compares the posted number against that height, with margin.

What should I do at a low-clearance sign that is close to my height?

Stop before the structure and reroute. Creeping under to test it is how bridge strikes happen; the sign is the decision, not the start of an experiment.

Why is car GPS dangerous for trucks?

It routes by distance and time with no knowledge of your height or weight, so it happily sends a thirteen-foot trailer to an eleven-foot underpass. Commercial routing exists for this reason.

Can a posted clearance be wrong?

It can be tighter than posted after repaving or under packed snow, which is why manuals teach margin rather than exact-match arithmetic.

How do low-clearance questions appear on the CDL test?

As judgment scenarios: what to do when the posted clearance is at or near vehicle height. The tested answer is stop and reroute, never proceed slowly.