Who Invented the Traffic Light?: William Potts of Detroit


Despite widespread credit to Garret Morgan for a 1923 patent, traffic lights very closely resembling modern lights were in use in Detroit three years earlier.

The first historically recorded traffic light was installed at a busy intersection outside of Parliament in London in 1868. This was before all but the earliest steam automobiles, and was meant mainly for horse and wagon traffic. It only had two colors, red for stop, and green for proceed with caution. It was powered by gaslight gas, and had to be manually switched from green to red. After a little over a month in operation this traffic light exploded and caused some injuries to its operator. After that, traffic light inventors seem to have taken a long break until the 20th century. Henry Ford’s “a car in every driveway” made new traffic control almost inevitable.

Inventors set about trying to design new traffic lights, and there were U.S. Patents issued in 1918 and 1919 to two different inventors. One to James Hoge of Cleveland, Ohio who invented a two color red green electric traffic light. Others were also experimenting with adapting railway signals to road use, but William Potts, a policeman from Detroit, deserves the credit for inventing the modern three color traffic light. He rigged it from parts from three colors of railroad lights and some wire and switches. It began replacing policemen directing traffic at the corner of Woodward and Michigan Avenues in 1920. It could control traffic from four directions, and was very successful. Within a year from that first day Detroit had over a dozen traffic lights in operation. Potts does not seem to have patented his invention, and a 1923 patent to Garret Morgan was soon purchased by General Electric (GE) which then began building and marketing traffic lights for the national market. Morgan’s design did not significantly effect the evolution of the traffic light, but GE used his patent as justification to try and corner the market on traffic light sales.

Although interconnection of lights goes back to some early two color hand switched systems, interconnection, remote control, and eventually automation are the next important developments in traffic light technology. Traffic lights are just one more transportation technology which makes the world run more smoothly that is often taken for granted and rarely noticed (except at long lights where they may be noticed with irritation). Arrow turn signal lights became important as traffic volumes increased and controlling flow became a more complex problem. Despite widespread credit to Garret Morgan, William Potts appears to be the actual inventor of the traffic light.