The History of CHMSL.
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 its small rear window. The Oldsmobile Toronado from 1971-1978, and the Buick Riviera from 1974-1976 had dual high-mounted supplemental brake lights/turn signals as standard, and were located just below the bottom of the rear window, visually aligned with the conventional rear tail lights/brake lights/turn signals just above the rear bumper. These innovations were not widely adopted at the time. Auto and lamp manufacturers in Germany experimented with dual high-mount supplemental brake lamps in the early 1980s, but this effort, too, failed to gain wide popular or regulatory support.
Early studies 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 is credited with reducing collisions by about 5%.
In 1986, the United States National Highway Traffic Safety Administration and Transport Canada mandated that all new passenger cars have a CHMSL installed. A CHMSL was required on all new light trucks and vans starting in 1994. CHMSLs are so inexpensive to incorporate into a vehicle that even if the lamps prevent only a few percent of rear end collisions they remain a cost-effective safety feature.