Rules of the Road

Road Rules: What’s the law on brightness of brake lights?

Brake lights, as seen in this traffic jam in Raleigh, N.C., on Jan. 14, 2015, can vary in intensity by appearance. Washington state follows federal regulations that limit how bright they can be.
Brake lights, as seen in this traffic jam in Raleigh, N.C., on Jan. 14, 2015, can vary in intensity by appearance. Washington state follows federal regulations that limit how bright they can be. cseward@newsobserver.com

Question: I was following a newer Cadillac Escalade a few nights ago. At a stop sign, when the driver applied the brakes, I was blinded by the brake lights. This sounds like a minor complaint, but it was dark and I couldn’t see for several seconds. The RCW does not address the brightness of rear lighting. What gives?

Answer: On the face of it, you’re right. If you search the Revised Code of Washington you won’t find any statute that specifies a maximum brightness for brake lights. However, I can make a trail from state to federal law that will answer your question.

While you will find general vehicle equipment requirements in state law, the federal government regulates the details in the Federal Motor Vehicle Safety Standards. These details are what vehicle manufacturers have to comply with in order to sell a vehicle in the United States. FMVSS 108 covers vehicle lighting. If you have a degree in industrial engineering and an abundance of time you may find it an enjoyable read. For the rest of us, it’s enough to know that one of the charts in the code specifies a minimum and maximum intensity for stop lamps.

At the state level, RCW 46.37.320 requires that vehicle lighting and other safety equipment complies with the requirements of the Washington State Patrol. If we step over to the Washington Administrative Code, we’ll find in WAC 204-10.021 that the state patrol has adopted the federal standards for motor vehicle safety. So in effect, the state requirement for the maximum illumination of stop lamps is the same as the federal requirement; you just have to follow a trail of legal code to get there.

It makes sense that states adopt the federal standard. Can you imagine the manufacturing nightmare for car companies if each state had its own set of safety rules? This already exists in the global market. Most countries conform to a United Nations standard, but a few countries, including the U.S., have different standards. That, in part, is why you can’t easily import a vehicle manufactured for most other countries into the U.S.

I think that answers your question, but it doesn’t really solve your original problem of the excessively bright brake lights. Auto manufacturers have to follow a specific testing procedure to meet the federal standards for illumination, so assuming that the lights you saw were installed by the manufacturer, they meet the illumination requirements set by law.

However, LEDs perform differently than incandescent lights, so even though they comply with electronic sensors that measure them, the way we respond to them with our human eyeballs is much different. Outside of changing federal law, probably the simplest way to reduce the impact from bright LED stop lamps is to allow more distance between your vehicle and the one in front of you. Since light intensity is subject to the inverse square rule of exponential decay, doubling the distance will result in a fourfold reduction in brightness. Look at that, we even squeezed a science lesson into the end of this one.

Road Rules is a regular column on road laws, safe driving habits and general police practices. Doug Dahl is the Target Zero Manager for the Whatcom County Traffic Safety Task Force. Target Zero is Washington’s vision to reduce traffic fatalities and serious injuries to zero by 2030. Ask a question.

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