1. 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 below.) 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.
2. 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 below.) The SAE
identification code for combination clearance and marker lamps is PC.
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