When are lights required to be installed on a trailer?
Lights are required whenever a trailer is used on public roads. Every trailer must have tail lights, brake lights, turn signals, a license-plate light, and rear reflectors; larger and wider trailers also need side marker and clearance lights. They are essential from dusk to dawn and in low visibility.
The answer
Any trailer towed on public roads must be equipped with lights so other drivers can see it and read its signals — this is required at all times the trailer is on the road, and it is absolutely essential from dusk to dawn or whenever visibility is reduced (rain, fog, snow). The lighting must duplicate the signals of the towing vehicle so that the trailer, not just the truck, communicates braking, turning, and presence.
The universally required lights on essentially every road-going trailer are:
- Tail lights (red) — mark the rear and show the trailer is present at night.
- Brake lights (red) — illuminate when the tow vehicle brakes.
- Turn signals (amber/red) — indicate turns and lane changes.
- License-plate light (white) — illuminates the rear plate.
- Rear reflectors (red) — passive reflectors required even on small trailers.
Additional lights are required as the trailer gets larger, wider, or heavier:
- Side marker lights and side reflectors — required on longer trailers so the sides are visible.
- Clearance and identification lights — required on trailers over a certain width (commonly 80 inches / 2.03 m wide), showing the trailer's outer edges and, for wide loads, three identification lights at the top rear.
- Intermediate side markers — required on longer trailers (typically over 30 feet).
Why the common wrong answers are wrong
Several tempting but incorrect beliefs trip people up:
- 'Only at night' — Wrong. The lights must be installed and functional whenever the trailer is on the road; you cannot legally tow a trailer that simply lacks brake lights and turn signals even in daylight, because you may still need them for signaling and low-visibility conditions.
- 'Small trailers don't need brake lights or signals' — Wrong. Tail lights, brake lights, and turn signals are required on virtually all road-going trailers regardless of size; what changes with size is the addition of marker and clearance lights, not the removal of the basics.
- 'Reflectors are optional' — Wrong. Rear reflectors are required even on the smallest utility trailers.
- 'Trailer lights are never needed in the day' — Wrong. Brake and turn signals must function day and night; visibility equipment matters most at night but the requirement to have working lights applies whenever you tow.
The bigger picture
Trailer lighting requirements come from federal standards (FMVSS 108) and are enforced through each state's vehicle code, so exact thresholds — width triggering clearance lights, length triggering side markers, and weight triggering separate braking systems — vary by jurisdiction. The consistent principle is conspicuity and signaling: the trailer must be seen and must repeat the tow vehicle's stop and turn signals. Heavier trailers (often over 3,000 lb GVWR, varying by state) additionally require their own brakes and sometimes breakaway systems, which tie into the electrical/lighting harness. Before towing, always confirm every light works and matches the tow vehicle, and check your state's specific size and weight thresholds.
| Any trailer on public roads | Tail lights, brake lights, turn signals, license-plate light, rear reflectors |
| Longer trailers (roughly 30 ft+) | Add side marker lights and side reflectors, plus intermediate side markers |
| Wide trailers (about 80 in / 2.03 m or wider) | Add clearance lights and rear identification lights marking the outer edges |
| Heavier trailers (over state GVWR threshold) | Add trailer brakes/breakaway system wired into the harness |
| Night or low visibility | All of the above must be clean, functional, and lit |
Frequently asked
What lights are legally required on a trailer?
Every road-going trailer needs red tail lights, red brake lights, turn signals, a white license-plate light, and rear reflectors. Larger and wider trailers must add side marker lights, side reflectors, and clearance/identification lights. All must match and duplicate the tow vehicle's signals.
Do small trailers need brake lights?
Yes. Brake lights, tail lights, and turn signals are required on virtually all trailers used on public roads regardless of size. Being small does not exempt a trailer from the basic lighting that lets following drivers see when you brake or turn; only the additional marker and clearance lights depend on size.
When do trailers need side marker lights?
Side marker lights and side reflectors are generally required on longer trailers, with intermediate side markers added on trailers over about 30 feet. They make the length of the trailer visible from the side at night. Exact length thresholds are set by federal standard FMVSS 108 and your state's vehicle code.
Are trailer lights required during the day?
Yes — the lights must be installed and fully functional whenever the trailer is towed, day or night. Brake lights and turn signals are needed for signaling at all times, and you cannot legally tow a trailer with missing or broken required lights even in daylight.