I don't remember the "discount"...
... but I seem to remember that an insurance company initially bought and installed them on cars to determine if they did indeed reduce the number of rear-end accidents...
Maybe even a fleet of cars or rentals. They were really guady and ugly, as well as attention-getting...
Wiki says:
Centre High Mount Stop Lamp (CHMSL)
LED CHMSL retrofitted on a 1974
Valiant
In
North America since 1986, in
Australia and
New Zealand since 1990, and in
Europe (with the exception of
Ireland) since 1998, a central brake lamp, mounted higher than the vehicle's left and right brake lamps and called a
Centre High Mount Stop Lamp (CHMSL), is also required. The CHMSL (pronounced
/ˈtʃɪmzəl/) is also sometimes referred to as the
centre brake lamp, the
third brake light, the
eye-level brake lamp, the
safety brake lamp, the
high-level brake lamp, or the
Liddy Light (for
Elizabeth Dole, who as
U.S. Secretary of Transportation presided over its introduction in the United States
[50]). The CHMSL may produce light by means of a single central filament bulb, a row or cluster of filament bulbs or
LEDs, or a strip of
Neon tube.
The CHMSL is intended to provide a deceleration warning to following drivers whose view of the vehicle's left and right stop lamps is blocked by interceding vehicles. It also helps to disambiguate brake vs. turn signal messages in North America, where red rear turn signals identical in appearance to brake lamps are permitted, and also can provide a redundant brake signal in the event of a brake lamp malfunction. The CHMSL is required to illuminate steadily; it is not permitted to flash except in certain cases under severe braking.
[51][52]
On passenger cars, the CHMSL may be placed above the back glass, affixed to the vehicle's interior just inside the back glass, or it may be integrated into the vehicle's deck lid or into a
spoiler. Other specialised fitments are sometimes seen; the
Jeep Wrangler and
Land Rover Freelander have the CHMSL on a stalk fixed to the spare wheel carrier. Trucks, vans and commercial vehicles sometimes have the CHMSL mounted to the trailing edge of the vehicle's roof. The CHMSL is required by regulations worldwide to be centred laterally on the vehicle, though
ECE R48 permits lateral offset of up to 15 cm if the vehicle's lateral centre is not coincident with a fixed body panel, but instead separates movable components such as doors.
[15] The
Renault Master van, for example, uses a laterally offset CHMSL for this reason. The height of the CHMSL is also regulated, in absolute terms and with respect to the mounting height of the vehicle's conventional left and right brake lamps.
[53] Depending on the left and right lamps' height, the lower edge of the CHMSL may be just above the left and right lamps' upper edge.
[edit] History
The 1968–1971
Ford Thunderbird could be ordered with additional high-mounted brake and turn signal lights.
[citation needed] 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,
[54] 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%.
[55]
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.
[55]
