Not a twentieth-century invention
The first traffic signal predates the car. It was built to stop horse-drawn traffic from endangering pedestrians, and the people who designed it borrowed their design from railway signals. The date usually given is December 10, 1868, and the place is Parliament Square in Westminster, London, near the Houses of Parliament.
The Westminster semaphore, 1868
The device is credited to John Peake Knight, a railway signaling engineer. It looked like a railway semaphore: a post with movable arms. By day, the arms were held out horizontally to signal “stop” and lowered to about forty-five degrees to signal “caution.” By night, the signal used lamps, with a red light for stop and a green light for caution, lit with gas. A police officer operated it with a lever at the base.
The reason for it was traffic congestion in the area, where carriages, carts and walkers crossed one another. It was an unusual experiment, installed with the backing of a railway signaling company’s expertise. The invention went wrong in the next weeks. The gas lantern is reported to have exploded in early January 1869, injuring the police officer who operated it. The signal was then removed, and the idea faded for decades. The exact details of the accident vary in the accounts, so treat the specifics with caution.
The gap
Traffic signals did not spread immediately for a simple reason: there was too little traffic to justify them. Mounted police officers and constables on foot directed vehicles at busy junctions, using hand signals or flags. Only when motor vehicles multiplied in the early twentieth century did the need become acute. A policeman standing in the road was in danger from cars, and he could not do the job at night as easily.
Electric signals in the United States
Several American cities experimented in the 1910s, and each has some claim to a “first”:
- Salt Lake City, Utah, 1912. A police officer named Lester Wire is often credited with an electric red and green signal on a street corner. Evidence for the date and details is slim, and some historians treat the story with caution.
- Cleveland, Ohio, 1914. The Municipal Traffic Lighting Company, associated with James Hoge, installed an electric signal at Euclid Avenue and East 105th Street in August 1914. It used red and green lights and a buzzer to warn of a change. Hoge later received a patent, in 1918. Cleveland’s signal is commonly the best-documented early electric one.
- Detroit, 1920. William Potts, a police officer, is credited with developing a four-way, three-color signal, adding amber to red and green, which resembles the modern idea of a signal that warns before changing.
Each of these used different technology. Some were operated by hand and others by a mechanical timer, and early lamps were not bright. It is not a clean sequence, and writers choose among them depending on how they define “first.”
Garrett Morgan
Garrett Morgan, an African American inventor in Cleveland, is widely credited with a traffic signal patented in 1923. His design was a T-shaped pole with a mechanism that could halt traffic in all directions for a moment, giving pedestrians time to cross. The common story says he sold the rights to the General Electric Company. Some popular accounts overstate his role, calling his the first signal or the origin of the three-color light, which is not accurate, since earlier electric signals already existed. A more careful statement is that Morgan patented a particular design and is part of the history, and that the patent is documented.
Making signals automatic
The earliest electric signals needed a person to flip a switch. Over the 1920s, automatic timers and then coordination between adjacent intersections reduced the need for an operator. Systems in the mid-twentieth century added sensors that could detect waiting cars, and later computers coordinated whole networks of signals. The principle of automatic switching, in which a simple input is turned into a defined output, links the story to other technologies.
Standardizing the colors
Red for stop and green for go came from railroad practice, where red had long been the color of danger. Amber was added as a transitional signal. National and international conventions in the following decades fixed the order of colors, their arrangement and the meaning of flashing signals. The arrangement is not identical everywhere. Some countries use a red and amber combination before green, for example, and that is a regional variation.
The adoption of electric signals was also helped by the spread of electric lighting more generally. Lamps became brighter and more reliable, allowing signals to be seen in daylight. Later lamps used light emitting diodes, which last longer and use less power.
A fair summary
- The earliest traffic signal was the gas-lit semaphore in London in 1868, designed by John Peake Knight.
- Electric signals appeared in several American cities between 1912 and 1920, and the credit is shared among people such as Lester Wire, James Hoge and William Potts.
- Garrett Morgan’s 1923 patent is well documented but is often described with exaggeration.
The traffic light, in other words, did not have one inventor. It grew out of railway signaling, the arrival of motor traffic and a series of local experiments, and its history is better told as a progression than as a single moment.