Under California law, trailers must have "reflectors, tail lights, brake and license plate lights." Generally, trailers are required to have two tail lights. The exception is for trailers and semitrailers manufactured after July 23, 1973, that are less than 30 inches in width. These trailers may have one tail lamp as long as it is mounted near the center of the vehicle. If two tail lamps are required, they must be mounted on the left and right sides of the vehicle at the same height--generally between 15 and 72 inches. Tail lamps must be red and visible in most cases from 1,000 feet to the rear.
If the lights on the vehicle you are towing are hidden, lamp-type turn signal lights are a must on the trailer. The signal lights should be mounted no lower than 15 inches and should be visible day and night from a distance of at least 300 feet. If the trailer requires clearance lights, the signal lights should be visible for at least 500 feet. Lights projecting to the rear should flash red or amber. If the lights should stop working while the vehicle is being driven, hand and arm signals must be used.
If a trailer is over 80 inches wide, it is required to have clearance lights. At least one red clearance light is required to be mounted on each side of the rearward-facing portion of the trailer at the extreme width of the trailer. The lamps must be visible from all distances between 50 and 500 feet to the rear of the trailer.
