How vehicle lights moved from fuelled lamps to LED headlights, why colours are fixed by international convention, and what each lamp is for on the road.
Automotive lighting is the set of functional lamps on a vehicle, including headlights, daytime running lights, turn signals, brake lights and reversing lights. Most colours and intensities are fixed by international rules from the 1949 Geneva and 1968 Vienna conventions. LED technology has since replaced many bulbs, offering faster response and longer life.
Automotive lighting covers the functional lights fitted to the front, rear, sides and sometimes the top of a vehicle. Some devices light the road for the driver while making the car visible to others, and others show turning, slowing or stopping. The colours and performance of most lamps are set by international regulations.
How did vehicle lighting develop?
Early road vehicles used fuelled lamps before electric lighting arrived. The first Ford Model T used carbide lamps for its headlights and oil lamps for its tail lights, and it did not have all-electric lighting as standard until several years after its introduction. Dynamos for headlights appeared around 1908 and were common on 1920s automobiles.
Trafficators, which were signalling arms that flipped up, came in the early 1900s and were operated mechanically or pneumatically. Tail lights and brake lights followed around 1915, and low-beam or dipped headlights were available by 1919. Self-cancelling turn signals were developed in 1940, the same year that sealed beam headlights became the only acceptable type in the US.
Sealed beam headlights had been introduced in 1936. By 1945 headlights and signal lights had been integrated into the body styling, which made the lamps part of the design rather than add-ons. Halogen headlights followed in Europe in 1960, and high-intensity discharge headlights arrived from 1991.
Colour and intensity rules turn a handful of lamp types into one shared language on every road.
What colours are lights allowed to show?
Vehicle light colours are largely standardised by convention. The rules trace back to the 1949 Geneva Convention on Road Traffic, and the 1968 United Nations Vienna Convention built on them. With some regional exceptions, rearward lights show red, side-facing lights and all turn signals show amber, and forward lights show white or selective yellow.
No other colours are permitted except on emergency vehicles. Countries outside the conventions, such as the United States and Canada, set different rules for side marker lights and for some turn signals, so the same lamp can look different on either side of a border.
Colour quality also changes with age. Amber bulbs made from clear glass with a coating can flake or fade, causing a turn signal to emit white light instead of the required amber. Regulators are still developing a colour durability standard, and no test protocol has been specified yet.
How do low and high beams differ?
Low beams, also called dipped, passing or meeting beams, provide adequate forward and lateral illumination without dazzling other road users. High beams, called main, driving or full beams, give an intense, centre-weighted light with no particular glare control. They are suitable only when the driver is alone on the road.
UN ECE dipped beams have a sharp, asymmetric cut-off, with the half nearest oncoming drivers kept flat and low. The United States and Canada use FMVSS and CMVSS standards, which produce more glare for oncoming drivers and poorer illumination of the near side of the road.
UN ECE regulations also permit higher-intensity high beams than US and Canadian rules allow. The term driving beam dates from the early days of night driving, when opposing vehicles were rare, and it still appears in UN regulations.
What auxiliary lights do drivers fit?
Auxiliary lights include driving lights, front fog lights, cornering lights and spotlights. Driving lights provide extra long-range illumination and appear mainly on rally cars and on vehicles in regions with long unlit roads. Front fog lights give a wide, low, bar-shaped beam designed for poor visibility.
Rules on use differ from one country to another. Russia allows no more than three pairs of driving lights on a vehicle, while Paraguay requires them to be switched off and covered when a vehicle is used in urban areas. Australia permits fog lights only when the driver is facing fog, mist or other conditions that restrict visibility.
Fog lights are often misused. An SAE study in the United States found that more people used them inappropriately in dry weather than properly in poor weather. Where cornering lights are allowed, they light the side of an intended turn, and UN rules limit them to operation below 40 km/h (about 25 mph).
Why do cars have daytime running lights?
Daytime running lights, or DRLs, make a vehicle more visible in daylight. UN Regulation 48 required passenger cars and small delivery vans first type-approved on or after 7 February 2011 to carry them, and large vehicles such as trucks and buses have been covered since August 2012.
The EU requires functionally dedicated DRLs compliant with UN Regulation 87, rather than letting the headlights or fog lights perform the role. Under UN Regulation 87 these lights are white, at least 400 candela on the axis, and capped at 1200 candela in every direction. In the US, DRLs may be amber or white and may produce up to 7,000 candela, which has drawn a large number of glare complaints.
The UK once required a dim-dip device or running lights on cars first used from 1 April 1987. A dim-dip device operated the dipped beams at between 10% and 20% of normal intensity, to give a town beam after dark. The European Court of Justice quashed the requirement in 1988, but dim-dip functionality remained on many new cars well into the 1990s.
How are turn signals and stop lamps designed?
Turn signals flash at a steady rate between 60 and 120 times a minute, and UN rules require them to be amber. Stop lamps are red and must be brighter than rear position lamps, and they are fitted in pairs. Reversing lights, rear fog lamps and the centre high-mount stop lamp complete the rear lighting in most markets.
| Lamp | Colour | Typical requirement |
|---|---|---|
| Front position lamp | White; amber on some motorcycles and in some countries | Low-intensity light for night conspicuity |
| Turn signal | Amber, or red at the rear in some markets | Flashes 60 to 120 times per minute |
| Stop lamp | Red, brighter than the rear position lamp | Fitted in multiples of two; 60 to 185 candela under UN rules |
| Rear fog lamp | Red | 150 to 300 candela under UN Regulation 48 |
| Reversing lamp | White | One or two lamps |
| Centre high-mount stop lamp | Red | Mounted higher than the stop lamps |
| Identification lamp | Amber at the front, red at the rear | Required on vehicles over 80 inches (2.032 m) wide in the US and Canada |
The centre high-mount stop lamp became mandatory in the US and Canada from 1986, in Australia and New Zealand from 1990, and in Europe from 1998 under UN Regulation 48. Early studies of taxis and fleet vehicles found that a third high-level stop light cut rear-end collisions by about 50%, although that benefit fell as the lamp became common, leaving an enduring reduction of 4.3%.
Turn signals have a long history of their own. Electric turn signals date from as early as 1907, and the silent film star Florence Lawrence is credited with a signalling arm in 1914, though she did not patent it. The modern flashing turn signal was patented in 1938 and became mandatory in 1967, after most major manufacturers had offered it as an option.
What does an emergency stop signal do?
An emergency stop signal makes the brake lights or hazard lights flash in phase at 3 to 5 Hz under heavy braking. It activates above 50 km/h (31 mph) when the emergency braking logic is triggered and deceleration is high enough. In February 2019, members of the European Parliament approved making it mandatory on new vehicles sold in the EU.
Toyota, Mercedes-Benz, Volvo and BMW were among the earlier makers to fit the feature. Other systems take different approaches. Some BMWs have adaptive brake lights that grow in size under severe braking, and some Volvo models make the stop lights brighter. Studies of vehicles with emergency stop signals have not yet shown a significant improvement in safety.
How have LEDs changed automotive lighting?
LEDs offer a very long service life, strong vibration resistance, high colour durability and high efficiency, and they can be packaged far more compactly than bulbs. When power is applied, LED stop lamps reach full intensity around 250 milliseconds faster than incandescent bulbs.
That speed has not been shown to make cars with LED stop lamps less likely to be struck from behind.
The first LEDs appeared in centre high-mount stop lamps on the 1986 Chevrolet Corvette C4. The Maserati 3200GT, introduced in 1999, was the first production car with full LED taillights, and the 2000 Cadillac DeVille was the first US passenger car with LED taillights. The 2007 Audi R8 used LED daytime running lights, and its optional LED headlights, made by AL-Automotive Lighting, were the world's first.
Full LED rear lighting was held back by the reverse light, because white LEDs did not yet exist. Blue LEDs solved the problem: mixing red, green and blue LEDs behind a lens in a defined pattern produced white light that met the legal requirements. LED systems remain sensitive to heat, so makers test thermal design and UV resistance, and LED signal lights must still meet intensity requirements after one minute and after thirty minutes of continuous operation.
What lights do emergency and service vehicles use?
Emergency vehicles carry intense warning lights of particular colours, usually rotating beacons, xenon strobes or arrays of LEDs. In most countries blue and red lights are used on police, fire and medical vehicles, while amber is used on tow trucks, construction vehicles and other special-service vehicles.
The US is strict about this. A forward-facing red light, flashing or steady, is reserved for official emergency vehicles, and a driver who sees one coming must yield. In the UK, doctors may use green warning lights, although these do not give any exemption from road traffic regulations, unlike the blue lights of statutory emergency services.
Taxis in many places carry roof-mounted lights, which can include an illuminated Taxi sign, a light to show that the cab is ready to take passengers, and an emergency panic light for use in a robbery.
What lights are inside and on the body?
Most cars have at least one dome light, which often switches on when a passenger door opens. Many vehicles keep it lit after the doors close, so passengers can fasten their seatbelts, and some fade it out slowly over a couple of minutes, a feature known as theatre lighting.
Some vehicles have approach or puddle lights in the mirrors or door edges, and many include lights in the boot, glovebox and engine compartment. Distributive lighting sends light from a single source through optical fibres or light guides to dashboards, door locks and cup holders.
