Why Does Red Mean Stop and Green Mean Go?

Why do we know to automatically stop at a red traffic light and go at a green light? The answer involves a train crash, a gas explosion, and the Model T Ford.

Railroad Signals

In the 1830s, the railroad industry developed a system of signals that would direct train engineers to stop or go. Like modern traffic lights, they used three lights to signal which action the trains should take. Red, the color of blood, has been a signal of danger for thousands of years. It easily lent itself to being the color for stop. At this time, green was chosen as the color for caution, and white was the color for go.

Soon, it became apparent that white was a bad choice. It was easily confused with other white lights. In 1914, a red lens fell out of a light fixture and left it shining white light, turning a stop signal into a go signal. A train zoomed through the white signal and crashed into a train going the opposite direction. To prevent similar incidents, green was reassigned to mean go, and yellow was chosen to represent caution. Yellow was different enough from the other two colors that it stood out and was readily visible to train engineers.

Around the same time the colored light signals were developed, railroads began using a mechanical signaling system called a semaphore. These were poles with an attached arm that pivoted to different positions to signal train drivers. Today, most countries have phased out semaphores in favor of colored lights. The term “semaphore” is now also used as a synonym for a traffic light and as a more general term for any visual signaling system. It comes from the Greek sema (“sign” or “signal”) and phoros (“bearer”), literally meaning “a bearer of signals.”

A railroad semaphore.
Photo by David Ingham, June 8, 2008, CC BY-SA 2.0 via Wikimedia Commons.

To the Streets

In 1865, London faced a growing problem with clashes between horse-drawn traffic and pedestrians in the streets. Railway manager and train engineer John Peake Knight, who specialized in building signaling systems for British railways, presented a lighted signaling system to the Metropolitan Police as a solution for controlling traffic and preventing accidents. His design combined a semaphore for use during the day and a system of red and green gas-powered lights for the night.

On December 10, 1865, Knight’s semaphore/light signal was implemented at an intersection near the Parliament building. It worked great, but only for about a month, when a leak in the gas line supplying the lights caused an explosion. The police officer operating the semaphore was badly burned, and the semaphore/light system was immediately discontinued.

The Model T

In 1913, the year the Model T Ford was rolled out, about 4,000 people died in car crashes in the United States. Roads and highways were simply not designed for vehicles that could travel at 40 miles per hour (which was lightning fast compared to about 15 miles per hour max for horse-drawn carriages). As the Model T made cars more affordable for the middle class, more and more people were driving on the road. Soon, crowded intersections became confusing and dangerous for motorists, pedestrians, horses, and cyclists competing for the right of way.

Police officers stationed in traffic towers manually signaled drivers using lights, semaphores, or their arms. When they used lights, red meant stop and green meant go, but they did not use a yellow light—instead, they blew a whistle to alert drivers that they were about to change the signal. However, few drivers paid attention, especially at busy intersections, and crashes continued to occur.

The Electric Traffic Light

With the growing use of electric lights rather than gas lamps in the late 1800s, the stage was set for the invention of the electric traffic light. The first in history was Lester Wire’s 1912 handmade contraption in Salt Lake City. A police officer exasperated with a growing number of traffic incidents in the city, Wire constructed a wooden box that looked like a birdhouse, set red and green lights in it, and raised it up on a ten-foot pole. While Wire was truly the first to invent the electric traffic signal, he is often overshadowed by others who came later and had civil authorities and patents on their side.

In 1914, Cleveland engineer James Hoge also had the idea to borrow the red-green light system used by railroads. He suspended electric lights on a wire above the intersection of Euclid Avenue and East 105th Street in Cleveland, creating the first “municipal traffic control system.” Hoge’s traffic apparatus was similar to Knight’s and Wire’s in that a police officer had to sit in a traffic tower and switch the light every so often. Unlike these more rudimentary systems, however, Hoge’s caught on quickly.

A police officer in a traffic tower.
Photo by Olle Karlsson, July 24, 1953, Sweden, from Wikimedia Commons.

In 1920, Detroit police officer William L. Potts invented a traffic signal suitable for a four-way intersection that used all three colors present in the railway system—red, yellow, and green. However, Pott’s system still required someone to manually change the light, a tedious and costly way to manage traffic.

Automatic signals were thus in high demand, and several systems were invented throughout the 1920s. The first ones changed the color of the lights at timed intervals, but this meant that vehicles had to stop even when there were no other cars crossing the intersection. Charles Adler Jr. then invented a signal that could detect a car’s horn honking. To get the light to change, a car could honk its horn. Though the signal would not be able to change again for at least 10 seconds, this system was not popular because of all the noise from honking cars.

Later on, more efficient and less noisy systems were invented to sense when cars were present at an intersection and time the traffic lights accordingly. By the 1930s, traffic lights were beginning to spread to other countries in the world, becoming a signal of progress, growth, and industry in the US and abroad. Additionally, in 1935, the various systems in use in the US were standardized by the federal government, and all cities with stoplights were required to adopt the red, yellow, green light system to avoid confusion and inconsistency from one city to the next. The colors also must be lined up in the order red, yellow, green from top to bottom (which also helps colorblind drivers to distinguish which light is on).

How effective was the traffic light at preventing accidents? Smithsonian Magazine explains that due to the traffic light, “motor vehicle fatality rates in the United States fell by more than 50 percent between 1914 and 1930” (Nelson, 2018).

Socialization, Semiotics, and Gamification

Unfortunately, the traffic light also contributed to the rise of road rage on the streets. As the Smithsonian Magazine notes, pedestrians and drivers no longer had to acknowledge one another at intersections; they merely waited until the lights signaled they could go. Patience wore thin as people began to grumble when waiting for red lights to change.

From a young age, the public began to be socialized into the knowledge of traffic light signals along with models for good citizenship. As early as 1919, a schoolteacher in Cleveland came up with a game called “Red Light, Green Light,” which taught children to recognize traffic signals. The red/yellow/green system soon became instinctual as people learned the simple set of actions they represented. In the words of communication experts, “our ability to respond appropriately” to conventions like the traffic light “depends on our ability to use cultural experience to interpret signs and symbols appropriately, instantly, and instinctively.” The study of these types of symbols is called semiotics. To interpret symbols like the traffic light, “humans rely on signifiers and message shortcuts, whose meanings develop over time into almost universally accepted aspects of language” (Rackham and Gray, 2021). Where the traffic light is a signifier, we don’t have to be told to stop when the light is red—we have a message shortcut that bridges the gap between seeing a red light and knowing to stop.

Soon, the red/yellow/green light scheme was ingrained in many aspects of culture, an assumed semiotic system that permeated many different areas of life. Traffic signals were incorporated into other children’s games and toys. Educational programs on everything from nutrition to healthy relationships use green, yellow, and red to signify when to proceed with an action, when to slow down or use caution, and when to stop. Your boss may give you the “green light” to proceed with a project, or you might receive a “yellow light” while negotiations are on hold.

Yellow: The Color of Ambiguity

While the green light is unambiguously a signal to go and the red light unequivocally means stop, the yellow light is—well, somewhat up to interpretation and context. Some drivers see it as a sign to slow down and prepare to stop, while others see it as an indication to speed up and get through the light before it turns red. Laws regulating yellow lights are intentionally vague, as drivers are expected to use their own discretion and common sense to navigate an intersection at a yellow light. Semiotics is not always so simple, it seems.

BONUS: Click here for some interesting and unique crosswalk signals from around the world.


Adams, Cecil. “Who Decided Red Means ‘Stop’ and Green Means ‘Go’?” The Straight Dope, March 7, 1986. https://www.straightdope.com/21341613/who-decided-red-means-stop-and-green-means-go.

History.com Editors. “First Electric Traffic Signal Installed.” This Day in History. History.com, August 3, 2020. https://www.history.com/this-day-in-history/first-electric-traffic-signal-installed.

I Drive Safely. “The History and Meaning of Colored Traffic Lights.” Retrieved May 29, 2021, from https://www.idrivesafely.com/defensive-driving/trending/history-and-meaning-colored-traffic-lights.

Marusek, Sarah. “Visual Jurisprudence of the American Yellow Traffic Light.” International Journal for the Semiotics of Law, 27, 183–191.https://link.springer.com/article/10.1007/s11196-013-9323-z.

Nelson, Megan Kate. “A Brief History of the Stoplight.” Smithsonian Magazine, May 2018. https://www.smithsonianmag.com/innovation/brief-history-stoplight-180968734/.

Rackham, Scott, and Paxton Gray. Social Media Communication. Orem, UT: MyEducator, 2021.

Scott. “The Origin of the Green, Yellow, and Red Color Scheme for Traffic Lights.” Today I Found Out, March 8, 2012. https://www.todayifoundout.com/index.php/2012/03/the-origin-of-the-green-yellow-and-red-color-scheme-for-traffic-lights/.

“Semaphore.” Online Etymology Dictionary. Accessed May 29, 2021, from https://www.etymonline.com/search?q=semaphore.

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