The world’s first known signal device for regulating street traffic came into being before the automobile was in use, back when traffic consisted only of pedestrians, buggies and wagons. It was installed in 1868 in London, at the intersection of George and Bridge Streets near the Houses of Parliament. Designed by railroad signal engineer JP Knight, it had two semaphore arms which, when extended horizontally, meant "stop"; and when drooped at a 45-degree angle, meant "caution." At night, red and green gas lights accompanied the "stop" and "caution" positions. Red meant "stop" and green meant "caution." The lantern, illuminated by gas, was turned by means of a lever at its base so that the appropriate light faced traffic. Traffic police officers operated semaphores and early traffic lights by hand. City officials didn’t think drivers would obey the signals if traffic officers weren’t there to enforce them. The traffic officers judged the traffic and decided when to change the signal. To alert traffic that the signal was about to change, they blew a whistle.

The first automobile traffic light was invented around 1912 by Lester Farnsworth Wire, who was then head of the traffic division of the Salt Lake City Police Department. His two lamps, one red and one green, came from lights then in use on seagoing vessels and railroad signals where they meant stop and go as they do now. This light was a large wooden box with two six-inch holes on each side. Inside the holes were Mazda lamps which had been dipped in red or green watercolors. The box was painted yellow and planted on top of a ten-foot pole. The light was installed in Salt Lake City in 1912 at the intersection of Main Street and Second South. It was operated by a patrolman who used a two way throw switch to change the light’s colors. To power the light, wires from the box were attached to the overhead trolley wires. At first the signal was a novelty and even a joke to the local community. No one wanted to stop for a “flashing bird house.” People stood on the corner just to watch it. Needless to say, Lester became very discouraged. However, a few citizens thought it was an improvement and wanted more placed around the city. People from larger cities were impressed by the light, but local residents thought it a curiosity and nuisance.

And with the coming of more automobiles, the situation got even worse. Police Officer William Potts of Detroit, Michigan, decided to do something about the problem. What he had in mind was figuring out a way to adapt railroad signals for street use. The railroads were already utilizing automatic controls. Potts used red, amber, and green railroad lights and about thirty-seven dollars worth of wire and electrical controls to make the world’s first automatic traffic light. It was installed in 1920 on the corner of Woodward and Michigan Avenues in Detroit. He actually invented several traffic light systems, including the overhang four-way system, but did not apply for patents.

The first person to apply for a patent to produce inexpensive traffic lights was Garrett Morgan, who received his patent in 1923. Garrett Augustus Morgan (1877-1963) realized the need to control the flow of traffic in Ohio. A gifted inventor and reportedly the first African American to own an automobile in Cleveland, Ohio, he invented the electric automatic traffic light. His traffic light was a T-shaped pole that had three positions: Stop, Go, and the third position allowed pedestrians to cross the street or road more safely. The reason for the third position was to halt traffic in all directions. Years later, this invention was sold to General Electric for $40,000. Signal devices similar to the traffic light were also patented for England and Canada. Morgan's hand-cranked semaphore traffic management device was in use throughout North America until all manual traffic signals were replaced by early models of the automatic red-, yellow- and green-light traffic signals currently used around the world. Shortly before his death in 1963, the United States Government awarded Morgan a commendation for his traffic signal.

"Traffic Light Tree" On a roundabout just beyond the Canary Wharf estate there are three trees, two are London planes; the third is a traffic light tree; Pierre Vivant's eternal tree replaced another London plane as it was dying. Funded and produced by the Public Art Commissions Agency, the tree is eight meters tall with 75 sets of lights, and it was installed in East London in 1999. The Traffic Light Tree was the winner in an international competition, and each set of lights has a cycle that is controlled by a computer. "The sculpture imitates the natural landscape of the adjacent London Plane Trees, while the changing pattern of the lights reveals and reflects the never ending rhythm of the surrounding domestic, financial and commercial activities," says Vivant. Born in Paris in 1952 Vivant has been commuting between his Oxford and Paris Studios since 1973 producing and exhibiting work on both sides of the Channel.

Red has often been the color chosen when the goal was to attract attention, since red, more than any other color, heightens nervous tension in people. Green, on the other hand, has a neutral effect on human emotions, so it was natural to use it to indicate an "okay to proceed" condition. When it was decided to add a caution lamp to the traffic light, yellow was chosen because, other than white, it was the color most distinguishable from red and green. White, of course, was not desired since it could be confused with the many other white lights—such as streetlights—that might be near the traffic light.