<P> Conspicuity for the rear of a vehicle is provided by rear position lamps (also called tail lamps or tail lights). These are required to produce only red light and to be wired such that they are lit whenever the front position lamps are lit, including when the headlamps are on . Rear position lamps may be combined with the vehicle's stop lamps or separate from them . In combined - function installations, the lamps produce brighter red light for the stop lamp function and dimmer red light for the rear position lamp function . Regulations worldwide stipulate minimum intensity ratios between the bright (stop) and dim (rear position) modes, so that a vehicle displaying rear position lamps will not be mistakenly interpreted as showing stop lamps, and vice versa . </P> <P> Red steady - burning rear lights, brighter than the rear position lamps, are activated when the driver applies the vehicle's brakes . These are formally called stop lamps in technical standards and regulations and in the Vienna Convention on Road Traffic, though informally they are sometimes called "brake lights". They are required to be fitted in multiples of two, symmetrically at the left and right edges of the rear of every vehicle . International UN regulations specify a range of acceptable intensity for a stop lamp of 60 to 185 candela . In North America where the UN regulations are not recognised, the acceptable range for a single - compartment stop lamp is 80 to 300 candela . </P> <P> In the United States and Canada since 1986, in Australia and New Zealand since 1990, and in Europe and other countries applying UN Regulation 48 since 1998, a central stop (brake) lamp mounted higher than the vehicle's left and right stop lamps and called a "centre high mount stop lamp (CHMSL)", is also required . The CHMSL (pronounced / ˈtʃɪmzəl /) is sometimes informally called the "centre brake lamp", the "third brake light", the "eye - level brake lamp", the "safety brake lamp", or the "high - level brake lamp". The CHMSL may use one or more filament bulbs or LEDs, or a strip of neon tube as its light source . </P> <P> The CHMSL is intended to provide a warning to drivers whose view of the vehicle's left and right stop lamps is blocked by interceding vehicles . It also provides a redundant stop light signal in the event of a stop lamp malfunction . In North America where rear turn signals are permitted to emit red light, the CHMSL also helps to disambiguate brake lights from rear position lights and turn signal lights . </P>

When did the third brake light become law