A Center High-Mounted Stop Lamp (usually seen abbreviated as CHMSL) is a third stop lamp, or brake light, mounted on the rear of a vehicle. It is usually placed above the rear window, or is affixed inside the window and projects through it. In some creative arrangements, the CHMSL is integrated into a spoiler. A CHMSL is usually thought of as a car safety feature.

The stop lamps on vehicles are traditionally placed in the same housing as the tail lights and turn signals. This confused other drivers, and reduced their reaction time while they determined if a vehicle is actually braking. This effect is worse in North America, where the same red lights are generally used for all three functions. There was a need to place an additional stop lamp on vehicles to supplement the two traditional lamps.

The 1968-1971 Ford Thunderbird could be ordered with additional high-mounted brake and turn signal lights. These were fitted in strips on either side of the (small) rear window. They were rarely specified. The Oldsmobile Toronado from 1971 had high-mounted supplemental brake lights as standard. These innovations were not widely adopted at the time.

In the early 1980’s, a study involving taxicabs and other fleet vehicles found that a third stop lamp reduced rear-end collisions by about 50%. The lamp’s novelty probably played a role, since today the lamp only reduces collisions by about 5%.[1] (http://www.nhtsa.dot.gov/cars/rules/regrev/evaluate/808696.html)

It is possible that today, familiarity with third brake lights has reached the extent that drivers may indeed not respond quickly enough if an older vehicle without a third brake light decelerates in front of them, since the familiar cue is not there.

In 1986, the United States NHTSA mandated that all new passenger cars have a CHMSL installed. Light trucks were required to have the lamps installed in 1994. CHMSL's are so inexpensive to incorporate into a vehicle that even if the lamps prevent only a few percent of rear end collisions they will pay for themselves in reduced damages. A later NHTSA study (http://www.nhtsa.dot.gov/cars/rules/regrev/evaluate/808696.html) shows that CHMSLs reduce rear-impact crashes by 4.3%.

CHMSL's, unlike most other automotive lamps, often employ light_emitting diodes. CHMSL’s are often difficult to access or manufactured in irregular shapes. The longevity and versatility of LED’s leads to their use in these situations.