Center high-mounted stop lamp

The centre high-mounted stop lamp, abbreviated CHMSL and often called the third brake light, is a single brake lamp positioned high and centrally at the rear of a vehicle to supplement the two conventional stop lamps mounted lower at each corner. It exists to address a simple but persistent road-safety problem: a following driver may not promptly register that the car ahead is braking, particularly in heavy traffic where the standard tail lamps can be obscured by the vehicle directly in front. A bright light placed near the following driver's natural line of sight, often at windscreen height, conveys the braking message faster.

Functionally the CHMSL is wired to illuminate only under braking, in parallel with the main stop lamps, and unlike the lower lamps it does not double as a tail or indicator light, so it is unambiguous: when it glows, the car ahead is slowing. Early units used filament bulbs behind a red lens, but the lamp lends itself naturally to light-emitting diodes, which switch on almost instantaneously and are slim enough to integrate neatly. It is commonly mounted inside the rear window, on a parcel shelf, on the bootlid or tailgate, or built into a roof spoiler.

The value of the lamp lies in reaction time and crash reduction. By giving following drivers an additional, centrally located and elevated cue, it shortens the interval before they begin braking, which directly reduces rear-end collisions. Research conducted before its adoption suggested a meaningful fall in such impacts, and although the benefit has moderated as the lamp became universal and familiar, it remains a cost-effective safety feature.

Its history is closely tied to regulation. Following extensive trials, the United States made the CHMSL mandatory on new passenger cars from the 1986 model year and on light trucks shortly afterwards, and many other markets subsequently required it. This regulatory origin explains why the lamp is now an essentially universal fixture on the world's vehicles rather than an optional styling element.

In practical terms the lamp is low-maintenance, though LED arrays can develop dead segments over time and in-window units may suffer from trapped condensation or a deteriorating gasket. Because it is a required item, a failed CHMSL can be a roadworthiness defect in jurisdictions that test for it, so it should not be ignored.

The CHMSL sits within a broader family of rear-signalling and conspicuity measures intended to communicate a vehicle's intentions to others, conceptually related to the brake-force display, which modulates the brake lights under heavy or emergency braking, and complementary to forward conspicuity aids such as daytime running lights.