DOT Requirements for Trailers: What the Rule Covers

DOT requirements for trailers are lighter than most people expect. The federal marking rule, 49 CFR 390.21, applies to the self-propelled vehicle, meaning the truck or tractor. A towed trailer does not have to carry your name and USDOT number, and FMCSA said when it wrote the rule that it did not think marking trailers was necessary (Federal Register).

A trailer still matters in two ways. Its weight counts toward your combination, which can decide whether your rig is a commercial motor vehicle at all. And plenty of carriers letter their trailers anyway, for their own reasons. This guide separates the rule from the habit.

What the marking rule covers

The rule says every self-propelled commercial motor vehicle must show the carrier's name and USDOT number on both sides (49 CFR 390.21). FMCSA's own news release on the rule describes it as the name and number on both sides of the power unit (FMCSA). In the final rule, FMCSA said it did not believe the extra marking of trailers was necessary (Federal Register, 2000).

The same section has a separate identification rule for intermodal equipment providers. That is a different kind of company from a carrier pulling its own trailer, so most readers can skip it.

For the full rule on the truck itself, see our USDOT number display rules.

Where a trailer still counts: weight

The federal definition of a commercial motor vehicle includes a "self-propelled or towed" vehicle used in interstate commerce with a gross vehicle or combination weight rating of 10,001 pounds or more (49 CFR 390.5T). The combination rating can be the sum of the power unit's rating and the towed unit's rating.

That means a trailer can be the thing that tips a lighter truck over the line. FMCSA guidance says a truck and trailer that are each rated under 10,001 pounds can still be subject to the safety rules when their ratings add up past that point (FMCSA guidance). The test uses ratings, not what you happen to be hauling that day.

If that sounds like your setup, read who needs a USDOT number and do pickup trucks need DOT numbers. When the combination is covered, the marking still goes on the truck, not the trailer.

Trailer unit numbers and ID numbers

The federal marking rule does not ask for a unit number or an ID on a trailer. Those are a carrier's own system, and the reason to add them is practical: so your driver, your yard crew and your dispatcher can tell two similar trailers apart without walking around to find a plate.

If you do number your trailers, keep it simple and consistent. A short number that can be read over the phone beats a long code. Put it on a flat, clean spot you can read from where a person actually stands, away from seams and rivets so the vinyl lies flat.

Our Trailer / Unit Number decal pair is made for this. For numbering ideas across a larger fleet, see fleet and truck number decals, or browse the trailer decals page.

The factory rating plate vs a weight decal

The weight rating that counts in the federal definition is the one the manufacturer specifies for the vehicle (49 CFR 390.5T). It is shown on the manufacturer's label. A decal that repeats the number does not change the rating, and it cannot replace the label.

So treat a weight decal as a convenience copy for people who read the outside of the vehicle. Never cover or remove the factory plate to make room for one. Some states have their own weight-marking rules, which we cover in GVWR and weight stickers, and you can add a number with the GVW / Weight decal.

VIN decals on trailers

The federal marking rule does not mention a VIN. If a customer, an insurer, a lender or your state asks you to show one on a trailer, ask them for the exact wording and where they want it, then copy it from the title or the manufacturer's label character for character.

We offer a VIN Decal for trucks and trailers. It is a simple text decal, so double-check every character before you order.

Before you order for a trailer

• Confirm whether your truck is a covered vehicle, using the weight test above, then mark the truck.
• Treat trailer numbers as your own system, not a federal duty.
• Ask your state agency if you run only inside one state, because intrastate rules can differ. Rules change, so confirm with FMCSA or your state.