Traffic lights consist normally of three signals, transmitting meaningful information to road users through colours and symbols including arrows and bicycles. The regular traffic light colours are red, yellow (also known as amber), and green arranged vertically or horizontally in that order. Although this is internationally standardised,[4] variations exist on national and local scales as to traffic light sequences and laws.[5]
The method was first introduced in December 1868 on Parliament Square in London to reduce the need for police officers to control traffic.[6] Since then, electricity and computerised control has advanced traffic light technology and increased intersection capacity.[7] The system is also used for other purposes, for example, to control pedestrian movements, variable lane control (such as tidal flow systems or smart motorways), and railway level crossings.
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The first system of traffic signals was installed as a way to replace police officer control of vehicular traffic outside the Houses of Parliament in London on 9 December 1868.[8] In the first two decades of the 20th century, semaphore traffic signals like the one in London were in use all over the United States with each state having its own design of the device. In many cases, it was controlled by a traffic officer who would blow a whistle before changing the commands on this signal to help alert travellers of the change.[9]
In 1912, the first electric traffic light was developed by Lester Wire, a policeman in Salt Lake City, Utah.[10] It was installed by the American Traffic Signal Company on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio.[11][12][13] The first four-way, three-colour traffic light was created by William Potts in Detroit, Michigan in 1920.[14] His design was the first to include an amber 'caution' light along with red and green lights.[15] Potts was Superintendent of Signals for the Police Department of Detroit. He installed automatic four-way, three-colour traffic lights in 15 towers across Detroit in 1921.[16][15][14] By 1922 traffic towers were beginning to be controlled by automatic timers more widely. The main advantage of the use of the timer was that it saved cities money by replacing traffic officers. The city of New York was able to reassign all but 500 of its 6,000 officers working on the traffic squad; this saved the city $12,500,000.[17]
The control of traffic lights made a big turn with the rise of computers in America in the 1950s. One of the best historical examples of computerized control of lights was in Denver in 1952. In 1967, the city of Toronto was the first to use more advanced computers that were better at vehicle detection.[7] The computers maintained control over 159 signals in the cities through telephone lines.[19]
The 1968 Vienna Convention on Road Signs and Signals Chapter III provides international standards for the setup of traffic signal operations. Not all states have ratified the convention. A three-colour signal head should have three non-flashing lights which are red, amber, and green, either arranged horizontally (on the side opposite to the direction of traffic) or vertically (with red on top). A two-colour signal head may be used in temporary operation and consists of red and green non-flashing lights. In both cases, all lights should be circular or arrow-shaped.[4] Permissible signals for regulating vehicle traffic (other than public transport vehicles) are outlined in Article 23:[4]
Popular in Vietnam and China, countdown lights are additional lights installed next to (or above or below) the main signal lights. The countdown light is displayed by a countdown number with different colors (usually red, yellow, blue), matching the color of the light on. When the light counts to "0" (or 1), the main light color immediately changes. Countdown lights may have zeros in the tens or none, some countdown lights may flash when getting ready to zero. Yellow lights can also have countdown lights, but most lights do not. Usually the countdown light has 2 digits, in case the time of the main light (usually the red light, rarely the green light) is longer than 100 seconds, depending on the type of light, the following possibilities may occur:
In South Korea, yellow light dilemma zone is not legally recognised. In other words, when yellow light is on, traffic may not pass the stop line or enter the intersection even if cannot safely stop when the light shows. This has been reaffirmed by the ruling of the Supreme Court of Korea in May 2024.[24][25] There are criticisms in South Korea say that this is unrealistic and unreasonable. In addition, this can cause multiple collisions due to sudden braking.
Without an all-red phase, cross-turning traffic may be caught in a yellow trap. When the signal turns amber (also known as yellow), a turning driver may assume oncoming traffic will stop and a crash may result. For this reason, the US bans sequences that may cause a yellow trap.[26] This can also happen when emergency vehicles or railroads preempt normal signal operation.[27] In the United States, signs reading "Oncoming traffic has extended green" or "Oncoming traffic may have extended green" must be posted at intersections where the "yellow trap" condition exists.[28][29]
The United States is not party to the Vienna Convention; rather, the Manual on Uniform Traffic Control Devices (MUTCD) outlines correct operation in that country. In the US, a single signal head may have three, four, or five aspects (though a single aspect green arrow may be displayed to indicate a continuous movement). The signals must be arranged red, amber, and green vertically (top to bottom) or horizontally (left to right).[30] In the US, a single-aspect flashing amber signal can be used to raise attention to a warning sign and a single-aspect flashing red signal can be used to raise attention to a "stop", "do not enter", or "wrong way" sign.[30] Flashing red or amber lights, known as intersection control beacons, are used to reinforce stop signs at intersections.[31] The MUTCD specifies the following vehicular signals:[30]
In the Canadian province of Quebec and the Maritime provinces, lights are often arranged horizontally, but each aspect is a different shape: red is a square (larger than the normal circle) and usually in pairs at either end of the fixture, amber is a diamond, and green is a circle. In many southern and southwestern U.S. states, most traffic signals are similarly horizontal in order to ease wind resistance during storms and hurricanes.[32] Japanese traffic signals mostly follow the same rule except that the green "go" signals are referred to as (ao), typically translated as "blue", reflecting a historical change in the Japanese language. As a result, Japanese officials decreed in 1973 that the "go" light should be changed to the bluest possible shade of green, bringing the name more in line with the color without violating the international "green means go" rule.[33]
A speed sign is a special traffic light, variable traffic sign, or variable-message sign giving drivers a recommended speed to approach the next traffic light in its green phase[35] and avoid a stop due to reaching the intersection when lights are red.[36][a]
Pedestrian signals are used to inform pedestrians when to cross a road. Most pedestrian signal heads will have two lights: a 'walk' light (normally a walking human figure, typically coloured green or white) and a 'don't walk' light (normally either a red or orange man figure or a hand), though other variations exist.[37]
Pedestrian countdown timers are becoming common at urban signal-controlled crossings. Where a pedestrian countdown is shown, it is normally used in conjunction with the flashing hand signal (in the US and Canada) or blackout period (UK), showing the amount of time remaining in seconds until the end of the flashing hand or blackout.[20][44] Pedestrian countdown timers do not significantly increase or reduce the number of red- and amber-light running drivers. Studies have found that pedestrian countdown timers do significantly improve pedestrian compliance over traditional pedestrian signals; however, results are mixed.[44][45]
South Korea has implemented special LED ribbon embedded to the ground for pedestrian crossing which allows pedestrian to be fully immersed to his smartphone experience. By this additional signal the smartphone user does not need to lift his sight from the device to know when it is safe to cross.
In some jurisdictions such as Australia, pedestrian lights are associated with a sound device, for the benefit of blind and visually impaired pedestrians. These make a slow beeping sound when the pedestrian lights are red and a continuous buzzing or fast beeping sound when the lights are green. In the Australian States of Queensland, New South Wales, Victoria, and Western Australia, the sound is produced in the same unit as the push buttons. In a circle above the button, the sound is produced and can be felt along with a raised arrow that points in the direction to walk.[46] This system of assistive technology is also widely used at busy intersections in Canadian cities. In the United Kingdom, the Puffin crossings and their predecessor, the Pelican crossing, will make a fast beeping sound to indicate that it is safe to cross the road. The beeping sound is disabled during the nighttime so as not to disturb any nearby residents.[47]
In some states in the United States, at some busy intersections, buttons will make a beeping sound for blind people. When the light changes, a speaker built into the button will play a recording to notify blind people that it is safe to cross. When the signal flashes red, the recording will start to count down with the countdown timer. In several countries such as New Zealand, technology also allows deaf and blind people to feel when lights have changed to allow safe crossing. A small pad, housed within an indentation in the base of the box housing the button mechanism, moves downwards when the lights change to allow crossing. This is designed to be felt by anyone waiting to cross who has limited ability to detect sight or sound. In Japan, a traffic light emits an electronic sound that mimics the sound of birdsong to help the visually impaired. Some traffic lights fix the order and type of sound so that they can tell which direction is a green light. In general, "Piyo" (peep) and "Piyo-piyo", which is a small bird call, and "Kakk" and "Ka-kakk", which is a cuckoo call, are associated with this system.[48] Some pedestrian crossings in Lithuania make a slow beeping sound indicating that the traffic light is about to turn off. 152ee80cbc
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