Before automobiles even swarmed over the roads and streets, there was a need to control traffic to avoid accidents and keep vehicles moving smoothly.

The current systems have developed over more than a century, and they’re poised to change once again as vehicles become more connected and traffic control moves toward AI and complex computer-driven systems.


A signal history

December 10, 1868 — The first gas-lit traffic lights were installed outside the Houses of Parliament in London. Proposed by British railway engineer J.P. Knight to control the traffic of horse carriages, gas lights were manually controlled by a police officer using semaphore arms. At night, gas-lit red and green lights were used, but still changed by a police officer. The lights became a safety hazard as they sometimes exploded and injured police officers.

1912 — A traffic control device was placed on top of a tower in Paris at the Rue Montmartre and Grande Boulevard, with 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.

1912 — As automobile traffic increased, American policeman Lester Wire designed the first electric traffic light. It was first installed in Cleveland, Ohio, on August 5, 1914, at the corner of 105th and Euclid Avenue.

1917 — First interconnected traffic signal system installed in Salt Lake City, with six connected intersections controlled simultaneously from a manual switch.

1920 — William Potts, a Detroit policeman, invented the first four-way and three-colored traffic lights. He introduced yellow lights to indicate the light would change soon. Detroit became the first city to implement the four-way and three-colored traffic lights.

1920 — Los Angeles installs five signals on Broadway manufactured by the Acme Traffic Signal Co. The signals paired “Stop” and “Go” semaphore arms with small red and green lights and bells that rang just before the flags changed.

1920s — In a predominately Irish neighborhood in Syracuse, New York, traffic lights were vandalized frequently. The Irish citizens objected to the red light on top, which they viewed as a symbol of British oppression of Ireland (represented by green lights at the bottom). City officials flipped the signals in that neighborhood to have green lights on top.

1923 — Garrett Morgan received a patent for an electric traffic signal. The African American inventor owned a sewing machine company in Cleveland and, after witnessing a horrific accident, worked on his automated traffic signal system. GE paid him $40,000 for the invention.

1928 — Charles Adler Jr. developed a sonically actuated traffic light. To operate it, drivers pulled up to a red light and honked their horns to make the light change. Installed in Baltimore, it was the first actuated traffic signal in the United States and served as the basis for modern traffic signals.

1929 — Adler also invented a pedestrian push button, which was installed in Baltimore—the first pedestrian-actuated signal.

1950s — Computerized detection used in traffic lights. A pressure plate was placed at intersections so computers would know that a car was waiting at the red light.

1960s — As computers improved, they could monitor traffic and change lights in an even more efficient way.

1990s — The countdown timer was introduced to traffic lights to help pedestrians know whether they have enough time to cross the road before the signal changes color.

2010s — Connected vehicles can communicate with traffic signals and other vehicles. This can vastly improve speed, timing, and efficiency at intersections—perhaps as much as 40 percent as more vehicles get connected, according to Washington State University research.

Future — Connected vehicles and AI-driven traffic control could lead to another color in the traffic signal: blue. “We need a new signal called blue phase, meaning you should follow the vehicle in front of you,” says civil engineer and traffic control researcher Ali Hajbabaie, explaining that connected vehicles can work better with a signal system to keep traffic moving while human drivers could follow them during the blue phase.


Further reading

Smart signals (WSM, Fall 2019)

Car Country: A Environmental History. Christopher W. Wells, 2014, University of Washington Press

Onramps and Overpasses: A Cultural History of Interstates. Dianne Perrier, 2009, University of Florida Press

Door to Door: The Magnificent, Maddening, Mysterious World of Transportation. Edward Humes, 2016, Harper.

Visions for Tomorrow’s Mobility (smart magazine)

Traffic light (Wikipedia)

Ready, Steady, Go: The evolution of traffic lights (Science ABC)

Charles Adler, Jr. (Wikipedia)