Every vehicle using the roads must adhere to lighting regulations no matter what they are; being truckers and hauling large loads, plus the crossing of state lines means there are a whole raft of regulations affecting truckers and big rigs.
Here are the lighting regulations in nutshell courtesy of Used Semi Trailers:
All vehicles on the road must be properly lit to enhance visibility and help avoid accidents and incidents. Semitrailer regulations are found primarily in the U.S. Department of Transportation regulations which you will be aware of from your CDL training. Failure to understand the regulations or to properly light your tractor trailer is likely to result in being violated by law enforcement and inspection officials.
Semi trailers at the back must have turn signals comprising at least 2 red or yellow signal indicators.
They must be precisely located between 15 inches and 83 inches off the level of the ground.
There must be two or more brake lights which must be red in color. These must be placed between 15 inches and 72 inches above the level of the ground.
The rear license plate must be illuminated by a white covered light – there are no positional requirements, except the light must be placed to allow the rear license plate to be read at night.
For semitrailers in excess of 80 inches in width, there must be at least 2 clearance lights which are red in color. They must be placed on each side of the semitrailer at the top rear. In addition, a set of three red identification lights should be centered on the rear top too.
There must be at least one red marker light towards the rear side. They must be as close to the back end of the semitrailer as possible and between 15 inches and 60 inches above the level of the ground. Semitrailers which are less than 80 inches do not have a maximum height restriction for setting side markers.
One yellow light marker should also be placed towards the front of the semitrailer, as close to the front as possible. This side marker should be over 15 inches above the ground with no maximum height.
If the trailer is greater than 30 feet in length, there needs to be a further, intermediate yellow light side marker on each side and placed as close to the center of the trailer length as possible, and again at least 15 inches above ground level with no maximum height.
The 80 inch width limit again applies – if greater than 80 inches, then there must be two front clearance lights which are yellow in color. They must be at the widest points of the trailer as far as is practicable and they must be as high above the ground as it practical.
Tanker trailers must also have these lights situated below the tank itself too.