Centre High Mount Stop Lamp (CHMSL)
In North America since 1986, in Australia and New Zealand since 1990, and in Europe 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). 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.
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. 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. 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.