Traffic lights are signalling devices positioned at road intersections, pedestrian crossings, and other locations to control flows of traffic. The history of traffic lights is associated with the historic growth of the automobile. Traffic lights were first introduced in December 1868 in London to assist police officers controlling traffic. Since then, electricity and computerised control has advanced traffic light technology and increased intersection capacity.
The origins of traffic signals Before traffic lights, traffic police controlled the flow of traffic. A well-documented example is that on London Bridge in 1722. Three men were given the task of directing traffic coming in and out of either London or Southwark. Each officer directed traffic coming out of Southwark into London, making sure all traffic stayed on the west side of the bridge. A second officer directed traffic on the east side of the bridge to control the flow of people leaving London and going into Southwark.
On 9 December 1868, the first manually-operated traffic signals showing a red or green light at night were installed outside the Houses of Parliament in London. They were installed to allow pedestrians, especially MPs hurrying to vote in Parliament, to cross Bridge Street. They were proposed by the railway engineer J. P. Knight of Nottingham who had adapted this idea from his design of railway signaling systems and constructed by the railway signal engineers of Saxby & Farmer. The design combined three semaphore arms surmounted by a gas lantern for night-time use. The entire apparatus was 22 feet (6.7 m) high. A police constable moved a crank which raised or lowered all three semaphore arms and rotated by 90° the red-and-green glass of the lantern (see cross-section). When the arms were horizontal, those in charge of vehicles and horses were to "Stop" clear of the crossing. When the arms were lowered to a 45 degrees angle, they could proceed with "Caution". At night the gas lantern at the top was lit so that it showed red lights to pedestrians and green lights to vehicles or vice versa. Although it was said to be successful at controlling traffic, its operational life was brief. It exploded on 2 January 1869 as a result of a gas leak and injured the policeman who was operating it.
Pre-electric signals Despite the failure of the world's first traffic light in London in 1869, by 1880, traffic lights spread all over the world. The early traffic lights in the late 19th century were very different from the ones that exist now. 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. One example was from Toledo, Ohio, in 1908. The words "Stop" and "Go" were in white on a green background and the lights had red and green lenses illuminated by kerosene lamps for night travellers and the arms were 8 feet (2.4 m) above ground. 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. The design was also used in Philadelphia and Detroit. The example in Ohio was the first time America tried to use a more visible form of traffic control that involved the use of semaphores. The device that was used in Ohio was designed based on the use of railroad signals. In 1912, a traffic control device was placed on top of a tower in Paris at the intersection of rue Montmartre and the boulevard Montmartre. This tower signal was operated by a policewoman and she used a revolving four-sided metal box on top of a glass showcase where the word "Stop" was painted in red and the word "Go" painted in white.
Electric signals In 1912, the first electric traffic light was developed by Lester Wire, a policeman in Salt Lake City, Utah. It was installed by the American Traffic Signal Company on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio. It had two colors, red and green, and a buzzer, based on the design of James Hoge, to provide a warning for color changes. The design by James Hoge allowed police and fire stations to control the signals in case of emergency.
In 1917, the first interconnected traffic signal system was installed in Salt Lake City, with six connected intersections controlled simultaneously from a manual switch.In 1920, the first four-way, three-color traffic light was created by police officer William Potts in Detroit, Michigan. He was concerned about how police officers at four different lights signals could not change their lights all at the same time. The answer was a third light that was colored amber, which was the same color used on the railroad. Potts placed a timer with the light to help coordinate the lights. A tower was used to mount the lights as the junction at which it was installed was one of the busiest in the world, with over 20,000 vehicles a day. In October 1920, Los Angeles installed its first automated traffic signals at five locations on Broadway. These early signals, manufactured by the Acme Traffic Signal Co., paired "Stop" and "Go" semaphore arms with small red and green lights. Bells played the role of today's amber lights, ringing when the flags changed—a process that took five seconds. By 1923 the city had installed 31 Acme traffic control devices.
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