Traffic lights, traffic signals, or stoplights – also known as robots in South Africa, Zambia, and Namibia – are signalling devices positioned at road intersections, pedestrian crossings, and other locations to control the flow of traffic. Traffic lights usually consist of three signals, conveying meaningful information to road users through colors and symbols, including arrows and bicycle symbols. The usual traffic light colors are red to stop traffic, amber to signal a change, and green to allow traffic to proceed. These are arranged vertically or horizontally in that order. Although this is internationally standardized, variations in traffic light sequences and laws exist on national and local scales. Traffic lights were first introduced in December 1868 on Parliament Square in London to reduce the need for police officers to control traffic. Since then, electricity and computerized control have advanced traffic light technology and increased intersection capacity. The system is also used for other purposes, including the control of pedestrian movements, variable lane control (such as tidal flow systems or smart motorways), and railway level crossings.
History
In December 1868, the first traffic signals showing a Red or green light at night were installed outside the Houses of Parliament in London. They were invented by John Peake Knight. A police constable raised or lowered the semaphore arms and, at night, operated a lever to control the lights which drivers and pedestrians saw. This system exploded on 2 January 1869 and was taken down. This early traffic signal led to other parts of the world implementing similar traffic signal systems. 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. These traffic signals were controlled by a traffic officer who adjusted the signals to direct traffic. In 1912, the first electric traffic light was developed by Lester Wire, a policeman in Salt Lake City, Utah. In August 1914, the American Traffic Signal Company installed it on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio. In 1920, the first four-way, three-color traffic light was created by William Potts in Detroit, Michigan. His design was the first to include an amber 'caution' light along with red and green lights. Potts was Superintendent of Signals for the Police Department of Detroit. In 1921, he installed automatic four-way, three-color traffic lights in 15 towers across Detroit. By 1922, traffic towers were being controlled by automatic timers more widely. The main advantage of using the timer was that it saved cities money by replacing traffic officers. New York City was able to reassign all but 500 of its 6,000 officers working on the traffic squad, saving the city $12,500,000. In 1923, Garrett Morgan patented a design of a manually operated three-way traffic light with moving arms. The control of traffic lights changed with the rise of computers in America in the 1950s. One of the best historical examples of computerized control of lights was in Denver, Colorado, in 1952. In 1967, Toronto, Canada, was the first to use more advanced computers better suited to vehicle detection. The computers maintained control over 159 signals in Toronto through telephone lines.
Vehicular signals
A set of lights, known as a signal head, may have one, two, three, or more aspects. The most common signal type has three aspects facing the oncoming traffic: red on top, amber ("yellow" in North America, "orange" in Australia) below, and green below that. Additional aspects may be fitted to the signal, usually to indicate specific restrictions or filter movements.
Meanings of signals 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 red, amber, and green lights. They should be arranged either vertically, with red on top, or horizontally, with red opposite the traffic flow. 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. Permissible signals for regulating vehicle traffic (other than public transport vehicles) are outlined in Article 23:
Green arrows are added to signals to indicate that traffic can proceed in a particular direction while the main lights for that approach are red, or that traffic can only proceed in one particular direction. Alternatively, when combined with another green signal, they may indicate that turning traffic has priority over oncoming traffic (known as a "filter arrow"). Flashing amber arrows typically indicate that road users must give way (to other drivers and pedestrians) before moving in the direction of the arrow. These are used because they are safer, cause less delay, and are more flexible. Flashing amber arrows will normally be located below the solid amber.
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