Does law require third brake light to be in working order?


The Center High Mounted Stop Lamp became required lighting equipment on new cars in 1986 and light trucks in 1994.

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Question: I have questions about lights. For years cars and trucks have been made with a third/center stop light. I see many vehicles on which this light no longer is working. Is this legal? Also, most newer cars come with daytime running lights. Are there any requirements to have these on?

Answer: The internet believes that the third brake light was invented by a San Francisco cab driver who had been rear-ended 12 times. He wired an extra brake light into his rear window and was never rear-ended again. There’s no actual evidence supporting that story, but there is a bit of overlap with the truth.

In 1974 psychologist John Voevodsky wondered if a third brake light would better alert following drivers to slow down. He tested his hypothesis by installing an extra light in the rear window of 343 taxi cabs in San Francisco. He then compared rear-end crash frequency of those with a control group not equipped with a third light. His study found that taxis with a third brake light had 60% fewer rear-end collisions than taxis in the control group. Larger studies conducted by the National Highway Traffic Safety Administration didn’t find as impressive results, but consistently showed that the third brake light reduced collisions.

The third brake light is officially called the Center High Mounted Stop Lamp (CHMSL), because the government is too serious to call it something awesome like the Cyclops Light. It became required lighting equipment on new cars in 1986 and light trucks in 1994. Along with that requirement, there’s a law mandating that the CHMSL “shall display a red light” when the brakes are applied. A non-functioning CHMSL is a violation of the law, but more importantly, fixing it is an effective way of reducing your chance of a rear-end crash.

Regarding your question about daytime running lights (DRLs), many cars equipped with them don’t have the option to turn them off. If you do have that option, it wouldn’t be illegal to turn them off, since in the U.S. we don’t have laws that even require cars to be equipped with DRLs. And for anyone unfamiliar with DRLs, they’re kind of a dim version of headlights that come on automatically whenever you’re driving.

DRLs are great for increasing the visibility of a car, but they have a problem. On some vehicles your DRLs and dash lights turn on automatically, but your taillights don’t come on unless you turn them on. While DRLs don’t provide adequate illumination for driving at night, in environments with lots of additional light (like city driving) it’s easy to think your lights are on, when in reality you’re invisible from behind.

While lots of folks might like the idea of making their own behind less visible, that’s not a good thing for your car.

As of September, Canada solved this problem by requiring all new cars with DRLs (which I guess is all cars in Canada since DRLs are required there) to also illuminate their taillights. The current U.S. standard allows but doesn’t require taillights to be illuminated when DRLs are activated. The solution, grammatically speaking, is to change one word in the law from “may” to “must”. Practically speaking, I’m sure it takes a lot more than that to revise the Federal Motor Vehicle Safety Standards.

Meanwhile, I know a simple solution: turn on your lights whenever you drive. As the weather and visibility turn for the worse, now is a good time to start the habit. Seeing is important, and so is being seen.

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Doug Dahl, Target Zero manager communications lead, answers questions about road laws, safe driving habits and general police practices every Monday.