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A traffic light or traffic signal is a signalling device positioned at a road intersection or pedestrian crossing to indicate when it is safe to drive, ride or walk, using a universal color code.

History
In the 1920s, after continued destruction of a standard traffic light in its Tipperary Hill Irish neighborhood, the City of Syracuse in the United States gave up and installed a traffic light with green on the top. The Irish had objected to the fact that "British" red was placed above "Irish" green.On 10 December 1868, the first traffic lights were installed outside the Houses of Parliament in London. They resembled railway signals of the time, with semaphore arms and red and green gas lamps for night use.

The modern electric traffic light is an American invention. As early as 1912, Salt Lake City policeman Lester Wire set up the first red-green electric traffic lights. On 5 August 1914, the American Traffic Signal Company installed a traffic signal system on the corner of 105th Street and Euclid Avenue in Cleveland, Ohio. Based on the design of James Hoge, it had two colors, red and green, and a buzzer to provide a warning for color changes. The first three-color traffic lights were introduced in New York and Detroit in 1920.

The first interconnected traffic signal system could be seen in Salt Lake City, Utah in 1917, with six connected intersections controlled simultaneously from a manual switch. Automatic control of interconnected traffic lights was introduced March 1922 in Houston, Texas.

The first automatic experimental traffic lights in England were deployed in Wolverhampton in 1927.

Garrett Morgan is sometimes mistakenly credited as the inventor of the traffic light. See.

Ampelmännchen traffic lights have come to be seen as a nostalgic sign for the former German Democratic Republic.

Technology
In the mid 1990s, cost-effective traffic light lamps using light emitting diodes (LEDs) were developed; prior to this date traffic lights were designed using incandescent halogen light bulbs. Unlike the incandescent-based lamps, which use a single large bulb, the LED-based lamps consist of an array of LED elements, arranged in various patterns. When viewed from a distance, the array appears as a continuous light source (unless closely examined).

LED-based lamps have numerous advantages over incandescent lamps; among them are:

Much greater energy efficiency
Much longer lifetime between replacement, measured in years rather than months. (Replacing traffic light bulbs is expensive)
Brighter illumination.
The ability to display multiple colors and patterns from the same lamp. Individual LED elements can be enabled or disabled, and different color LEDs can be mixed in the same lamp.
The operational expenses of LED-based signals are far lower than equivalent incandescent-based lights. As a result, most new traffic light deployments in the United States and elsewhere have been implemented using LED-based lamps; in addition many existing deployments of incadescent traffic lights are being replaced. (LEDs are also replacing incadescent lamps in many other applications, such as vehicle taillights). Many of the more exotic traffic signals discussed on this page would not be possible to construct without using LED technology.

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