All types of trailers, from boat trailers, campers and horse trailers to 18-wheeler rigs, and everything in between, must have proper lighting before heading out on the road. As the United States Department of Transportation's (DOT) National Highway Traffic Safety Administration has strict requirements for trailer lights, check the trailer's lighting system to ensure everything complies with federal laws before heading out on the open road.

Rear Light Requirements

A pair of red brake lights; a pair of red taillights, used to indicate the trailer's width; a pair of yellow or red turn signals; and a single white license-plate light must be present on the rear of all trailers in the United States. Most of the lights, except for the license-plate lamp, must be placed as far apart as possible at an asymmetrical height between 15 and 72 inches, according to the NHTSA. The license-plate light must be mounted directly above or to the side of the license plate.

Side Light Requirements

Sidelights, used to make other drivers aware of the trailer's presence and denote its length, must be placed in the same location on both sides of the vehicle. Rear side lights must be red and set as far back on the trailer as possible, between 15 and 60 inches high. Yellow front side lights must be placed as close to the front of the trailer as possible and be at least 15 inches tall. All trailers over 30 feet long must have intermediate side lights to make other drivers aware of the extended length. These yellow lights must be placed in the middle of each side of the trailer and be at least 15 inches high.

Wide Trailers

Trailers that are more than 80 inches wide must be equipped with additional lighting on the front and rear to indicate the trailer's width. Two red rear clearance lights must be placed at the trailer's widest point, as far apart and as high as possible. Three red identification lights must be centered on the back of the trailer, spread between 6 and 12 inches apart, and placed as high as possible. Front clearance lights must be put as high and as far forward as possible, at the trailer's widest point.

Additional Regulations

All trailer lights must be permanently attached to the trailer. Lights on the side of the trailer must be parallel to the trailer's centerline, while those on the front and back must be perpendicular to the center line, excluding lights that are installed at a photometrically certified angle. In addition to lighting regulations, the NHTSA requires all trailers be equipped with rear and side reflectors to indicate vehicle size and presence further.