There are dozens of laws regulating lighting on cars. Laws regulate the types, size and colour of lights on every kind of vehicle that operates on the public roadways. According to the Federal Highway Administration, more than 2.5 million miles were driven by motorists on more than 4 million miles of roadways in 1999, breaking down to 9,870 miles per capita. Considering that those miles are driven at all hours of the day and night, uniform lighting for vehicles becomes a necessary concern for both visibility and safety.
Early automobiles added headlights as an afterthought to light the way in the dark. As more automobiles were produced, cars' lighting, like their movements, needed regulation so that vehicles could be seen by other drivers. Early regulations required white headlights and red taillights, a requirement that still exists. Early drivers often strung what we call "aftermarket" lights in place and connected them to the car's batteries. The Society of Automotive Engineers, now SAE International, was founded in 1905 and pioneered the concept of automotive (and, later, aeronautical) technical standards, including standards for electronics and lighting production and use.
Car lighting regulations establish customary meanings and uses called "conventions." For example, light colours allow motorists to understand immediately whether they are driving toward another vehicle or following it. Blue or red flashing lights tell us that an emergency vehicle is approaching and that we should move over and stop to let it pass. Regulations control how far lights should illuminate and be seen, providing both visibility for the driver and warning for other motorists. Regulations make requirements uniform throughout a state or country so that a new system doesn't have to be learnt as motorists drive from place to place.
Regulations are issued and enforced at two levels of government--federal and state. At both levels, departments of transportation are responsible for adopting and enforcing regulations to protect motorists and the public from accident, injury and death due to defect or insufficiency in the design, manufacture or use of equipment. The federal rules concentrate on the manufacture of equipment while state codes govern its applications.
The Federal Motor Vehicle Safety Standard 108 (FMVSS 108), administered by the Department of Transportation's Federal Highway Administration (Federal Motor Vehicle Safety Standards section of the Code of Federal Regulations, 49 CFR 571.108) sets standards and specifications (based on SAE recommendations) for all vehicle lighting equipment. Standards are outlined for original and replacement lamps (headlights, taillights, turning and other exterior lights), reflective and miscellaneous equipment.
State departments of transportation are responsible for regulating placement number and uses of lighting equipment. Although each state makes its own regulations, rules tend to be uniform from state to state. Accepted in all states are rules like the prohibition of the use of red lights anywhere but the rear of the car, and reserving blue lights for use by emergency vehicles.
Most people never think about car lighting regulations. Vehicles bought in the U.S. must conform to original equipment specifications, and conventional replacement equipment bought from a reputable source is almost always compliant.
Very few adults ever consider adding "aftermarket" lighting under their vehicle or "tricking it out" with neon lights. Anyone who is considering such additions should check for the "USDOT" or SAE approval code on the box, check the state transportation code or consult a local police official before spending money on an expensive part they may have to remove.