If you run a commercial vehicle in interstate commerce, the Federal Motor Carrier Safety Administration (FMCSA) requires your US DOT number to be displayed on both sides of the power unit. Get it wrong and you risk fines at a roadside inspection — get it right once and you never think about it again.

One note on the citation, because it trips people up when they go looking for the rule themselves. Everyone calls this "390.21," and in conversation that is fine — but section 390.21 was suspended when FMCSA paused its Unified Registration System rollout, and the text actually in force is section 390.21T. The requirements are the same either way; FMCSA even titles its own guidance "Marking of CMVs and Intermodal Equipment (390.21T)." If you pull up 390.21 on eCFR and find it marked suspended, that is why, and 390.21T is the section you want.

What has to be on the truck

  • Your legal business name (or DBA name as registered with FMCSA).
  • The letters "USDOT" followed by your number.
  • Your MC number, if you operate as a for-hire carrier crossing state lines.

Size and legibility rules

The markings must be in a color that contrasts sharply with the background, be readable from 50 feet away in daylight, and stay legible for the life of the marking. In practice that means clean, high-contrast vinyl lettering — not a faded magnet that peels at highway speed.

Permanent vinyl vs. magnetic

FMCSA allows removable markings, so magnetics are legal. But for a vehicle that lives on the interstate, permanent cut-vinyl lettering is the safer call: it will not blow off, slide, or trap moisture against your paint. We cut both — tell us how the truck is used and we will point you to the right one.

Bring us your USDOT and MC numbers and we will lay out a compliant, sharp-looking USDOT decal set the same day in most cases.

What the rule does not cover

Two omissions catch people, and both are worth knowing before you order. There is no letter height in the federal rule — the standard is legibility from 50 feet in daylight, and the two-inch figure everyone quotes is a shop convention, not a regulation. And the rule reaches self-propelled commercial motor vehicles, not trailers: a dry van is not required to carry your name, your USDOT number or a unit number by any federal rule. We took both apart in how big DOT numbers need to be and does a trailer need a USDOT number.

Keeping it legible is part of the rule

The requirement is not only that the marking exists at the moment it is applied — it has to be kept legible. A set that has faded to grey, lost a character to a door ding, or been half-covered by a repaint is a finding, and it is the one most likely to be written up because it is visible from outside the truck without an inspector touching anything.

Because we cut in-house, a faded or damaged set is usually a spot repair rather than a full reorder — same vinyl, same colour, same layout, matched against what is already on the door.

What exactly has to appear on the truck?

The carrier's legal name or a single trade name as listed on the MCS-150, and the identification number preceded by the letters USDOT, on both sides of the self-propelled power unit, in letters that contrast sharply with their background and are readable from 50 feet in daylight while the vehicle is stationary.

Do the markings have to be on the doors?

The rule says both sides of the power unit, not the doors specifically. The door is simply where a flat, clean, visible panel usually is.

Are magnetic signs acceptable?

Yes — the rule permits a removable device that meets the same identification and legibility requirements. What matters is contrast and readability, not whether the marking is permanent.

My numbers have faded. Is that a problem?

Yes. The rule requires markings to be kept legible, not merely applied. A faded set is a citable condition and one of the easiest for an inspector to spot. Since we cut in-house, we can usually match and replace just the affected panel.