History of traffic lights

Before traffic lights, traffic police controlled the flow of traffic. A well-documented example is that on London Bridge in 1722.[3] Three men were given the task of directing traffic coming in and out of either London or Southwark. Each officer would help direct traffic coming out of Southwark into London and he made sure all traffic stayed on the west side of the bridge. A second officer would direct traffic on the east side of the bridge to control the flow of people leaving London and going into Southwark.

On 9 December 1868,[4] the first non-electric gas-lit traffic lights were installed outside the Houses of Parliament in London to control the traffic in Bridge Street, Great George Street, and Parliament Street.[5] They were proposed by the railway engineer J. P. Knight of Nottingham who had adapted this idea from his design of railway signalling systems[6] and constructed by the railway signal engineers of Saxby & Farmer. The main reason for the traffic light was that there was an overflow of horse-drawn traffic over Westminster Bridge which forced thousands of pedestrians to walk next to the Houses of Parliament.[7] The design combined three semaphore arms with red and green gas lamps for night-time use, on a pillar, operated by a police constable. The gas lantern was manually turned by a traffic police officer with a lever at its base so that the appropriate light faced traffic.[8] The signal was 22 feet (6.7 m) high. The light was called the semaphore and had arms that would extend horizontally that commanded drivers to "Stop" and then the arms would lower to a 45 degrees angle to tell drivers to proceed with "Caution". At night a red light would command "Stop" and a green light would mean use "Caution".[3] 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 leak in one of the gas lines underneath the pavement[9] and injured the policeman who was operating it.[10]

An early two-light traffic signal by White Horse Tavern in Hudson Street, New York. Image taken in 1961

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.[3]^: 22 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.[3]^: 23 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.[1]^: 382

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.[3]^: 33

In 1912, the first electric traffic light was developed by Lester Wire, a policeman in Salt Lake City, Utah.[11] It was installed by the American Traffic Signal Company on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio.[3]^: 27–28[12][13] It had two colours, red and green, and a buzzer, based on the design of James Hoge, to provide a warning for colour changes. The design by James Hoge[14] allowed police and fire stations to control the signals in case of emergency. The first interconnected traffic signal system was installed in Salt Lake City in 1917, with six connected intersections controlled simultaneously from a manual switch.[3]^: 32

The first four-way, three-colour traffic light was created by police officer William Potts in Detroit, Michigan in 1920.[15] 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 coloured amber, which was the same colour used on the railroad.[7] Potts also 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.[3]^: 35

Los Angeles installed its first automated traffic signals in October 1920 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.[16]

The control of traffic lights made a big turn with the rise of computers in America in the 1950s. Thanks to computers, the changing of lights made traffic flow even better thanks to computerised detection. A pressure plate was placed at intersections so that computers would know that a car was waiting at the red light.[3]^: 135 Some of this detection included knowing the number of waiting cars against the red light and the length of time waited by the first vehicle at the red.[3]^: 141 One of the best historical examples of computerized control of lights was in Denver in 1952. One computer took control of 120 lights with six pressure-sensitive detectors measuring inbound and outbound traffic. The control room that housed the computer in charge of the system was in the basement of the City and County Building.[3]^: 141 As computers started to evolve, traffic light control also improved and became easier. In 1967, the city of Toronto was the first to use more advanced computers that were better at vehicle detection.[3]^: 141 The computers maintained control over 159 signals in the cities through telephone lines.[3]^: 143

Countdown timers on traffic lights were introduced in the 1990s. Timers are useful for pedestrians, to plan whether there is enough time to cross the intersection before the end of the walk phase, and for drivers, to know the amount of time before the light switches. In the United States, timers for vehicle traffic are prohibited, but pedestrian timers are now required on new or upgraded signals on wider roadways. Some pedestrian timers can be used by motorists as well to know how much time remains in the green cycle, because often when the pedestrian timer reaches zero, the signal will simultaneously turn amber.