Clearance, Side Marker, and Identification Lamps
The federal government requires clearance, side marker and identification lamps on all vehicles greater than 80 inches in width. All of these lamps must meet the same photometric requirements, which call for light that is spread 45 degrees to either side of the lamp centerline. (See diagram.) The SAE identification code for these lamps is P2. The purpose of clearance lamps is to indicate the width and height of the vehicle, so these lamps must be mounted on the front and rear of the vehicle, as close to the top and sides of the vehicle as is practical. The purpose of side marker lamps is to identify the length of the vehicle, and they must be mounted as close to the front and rear of the vehicle as is practical. The purpose of identification lamps is to indicate to other drivers that the vehicle they are mounted on is a large vehicle. Three of these lights are required on the front, and three on the rear of the vehicle, centered and as close as possible to the top, and spaced between 6″ and 12″ apart.
Combination Clearance and Side Marker Lamps
One cost-cutting measure that is legally allowed is to combine the functions of the clearance and side marker lamps into a single lamp. This is done by mounting a lamp with a very wide light emission pattern on the upper corner of the vehicle. In order for these lamps to function properly, they must be mounted at a 45 degree angle. Combination clearance and marker lamps spread light 90 degrees to each side of the lamp centerline. When mounted on the corner of the vehicle at a 45 degree angle, this allows them to point light to the same angles as separate clearance and marker lamps would. (See diagram.) The SAE identification code for combination clearance and marker lamps is PC.