The History of Traffic Rules — 2-part series

• Part 1: Dividing the Road
• Part 2: Speaking in Light (this article)

The History of Traffic Rules Part 2: Speaking in Light — Why Traffic Lights Are Red, Yellow and Green

In the 1830s, uniformed men stood beside railway tracks in Britain. They were called “railway policemen,” and their job was simple. When a train passed, they timed it, and when the next train came they used hand signals to tell it how long ago the one ahead had gone by.^[1][2] An arm held straight out meant the line was clear, and standing at ease meant there was trouble ahead. The problem was that the man knew only how many minutes had passed since the previous train, not where that train was now. If the train ahead broke down and stopped, the one behind came on with no idea and ran into it.

The traffic light on the road branched off from exactly these uneasy hand signals and flags. And the fact that its roots lay in the railway still leaves a trace in the colors of traffic lights today.

From Hand Signals to Colored Lights: The Railway’s Trial and Error

In January 1841, British railway officials reached an agreement to standardize the meaning of colors: red for danger, green for caution and white for clear.^[1] That same year Charles Gregory, an engineer on the London and Croydon Railway, set up a semaphore signal at New Cross, with arms that rose and fell.^[3] By day it conveyed its meaning through the angle of the arm, and by night through the color of a lamp. An arm held horizontal meant stop, an arm lowered to 45 degrees meant caution, and an arm dropped fully into the post meant proceed.^[4]

The system had a flaw that was easy to overlook: “proceed” was a white light. A lamp naturally shines white, and colors are made by placing red or green glass in front of it. So what happens if the red glass falls out or breaks? The lamp that had signaled danger suddenly turns white, the color meaning “go.” The driver runs toward that light without a doubt in his mind. Sources on the history of British signaling explain that after a run of such accidents, the practice of using white as the proceed signal was abandoned.^[5] However, I could not confirm any record that singles out one particular accident as the decisive blow, so stories in which a single event changed everything should be read with caution.

The timing of the change is fairly clear. According to a source citing Michael Vann’s An Illustrated History of Signalling, the Great Northern Railway was the first British company to change its proceed signal from white to green, in 1876. In 1892 the Board of Trade required green for the proceed signal on new lines, and the following year the Railway Clearing House recommended dropping white.^[5] In the United States, the Chicago and North Western Railway changed white to green in its rule book in 1893, followed by the Pennsylvania Railroad and the New York Central in 1916. Some sources add that around 1918 the Interstate Commerce Commission banned white as a proceed signal.^[6] Once green meant “go,” a new color was needed for “caution,” and yellow took its place. Yellow caution signals were properly standardized in Britain in the early 1920s, and are said to have been made official by a report of a committee of the Institution of Railway Signal Engineers in December 1924.^[5]

Something similar happened at sea. Britain made ship lights compulsory with the Steam Navigation Act of 1846, and by an Admiralty regulation of 1848, sidelights with red on the left (port) side and green on the right (starboard) side became established. This system later became an international rule.^[7] Where the words “port” and “starboard” came from is covered separately in The Origin of Port and Starboard. The idea was that you could read the lights of an approaching ship to tell which side of it you were seeing. Why red became the color of danger is often explained as coming from the color of blood or fire, but no source establishes a definitive origin, so all we can say for certain is that railways and ships both used red to mean “stop.”

On a railway line, trains run in single file, so one color to separate “stop” from “go” was enough. Could the same method settle the problem left at the end of Part 1, namely who goes first at an intersection? The idea first appeared on a road in London in 1868.

The Exploding Traffic Light Outside Parliament

That was the moment a signaling system proven on the railway crossed over to the road. In December 1868, what is known as the world’s first road signal stood at an intersection near the Houses of Parliament in Westminster, London. Its designer, John Peake Knight, was a railway signal engineer.^[8] It used semaphore arms by day and red and green gas lamps by night, and a police officer operated it from below by turning a handle.^[8][9]

The signal did not last long. On January 2, 1869, a gas leak caused an explosion at the base of the signal, and the police officer operating it suffered severe burns to his face. The signal was removed not long afterward.^[9] In an age when horse-drawn carriages made up most of the traffic, there was also little sense that a signal was really needed. Traffic lights would have to wait until cars poured onto the roads before they regained momentum.

Who Was First? Competing Stories of Invention

The story of the traffic light in the automobile age features a different “first” in each country.

In 1912, Lester Wire, a police officer in Salt Lake City in the United States, is said to have placed a wooden box with red and green electric lights on all four sides atop a pole at an intersection (Main Street and 200 South). A police officer operated it by hand with a switch. But he never took out a patent, and the original was later lost.^[10] It is more accurate to say that Wire is remembered as the “first” thanks to later records and testimony.

What is firmly documented is Cleveland, Ohio, in August 1914. The American Traffic Signal Company installed an electric traffic signal at the intersection of East 105th Street and Euclid Avenue. It had red and green lights and a buzzer that sounded when the color changed, and it is associated with a design by James Hoge.^[9] Then in October 1920, William Potts, a Detroit police officer, put up a signal tower with red, yellow and green lights facing all four directions at the intersection of Woodward Avenue and Michigan Avenue.^[9][11] The three-color signal we see today is generally regarded as having taken shape at this point. Who invented the yellow light is disputed, with several claimants besides Potts.

In short, there was no traffic light “invented by one person.” It was pieced together bit by bit by a British engineer borrowing railway signals, police officers working the controls by hand, and companies that added electricity and timers. In 1923, the American inventor Garrett Morgan received a patent for a three-position signal, a design with an intermediate stage between stop and go that halted traffic in all directions and bought time to clear the intersection.^[9] To pick one and call it the first, you have to decide first what counts as first. The answer differs depending on whether you mean the first signal, the first electrically lit signal or the first with all three colors.

For traffic lights to earn their keep, they had to be freed from human hands. In 1917, Salt Lake City introduced a system that controlled six intersections from a single switch, and as automatic timers spread in the early 1920s, cities were able to redeploy the traffic police who had stood at every intersection. The traffic light was a tool for managing traffic, and at the same time one of the first cases of handing a job once done by a single police officer over to a machine.

One Signal for Many Countries: From Geneva to Vienna

As traffic lights spread, a new problem appeared: every country had its own shapes, colors and signs. As more cars crossed borders, international agreement became necessary, and in 1931 a convention to unify road signs was concluded in Geneva. Not many countries took part.^[12] Then in 1949 a United Nations conference in Geneva adopted a road traffic convention together with a protocol on road signs and signals.^[12]

Today’s international standard is the Convention on Road Signs and Signals, signed in Vienna in November 1968. It entered into force in 1978. It specifies that traffic lights use three colors, red, amber (yellow) and green, with red placed at the top or on the side away from the direction of travel. In some countries, red and amber lit together mean that green is about to come on.^[13]

But a standard did not automatically mean a worldwide standard. The convention now has more than 70 parties (71 to 75 countries, according to sources from 2024 to 2026), but the United States, Japan and China are not among them.^[13] The United States follows its own rules, the Manual on Uniform Traffic Control Devices, and maintains its own system in other respects too, such as making heavy use of text on its signs. In the arrangement of colors on traffic lights, most countries do use the same layout, but in signs and detailed rules, some regions still differ greatly.

The Light Is Green, So Why Is It Called “Blue”?

Now to the matter of color. The proceed signal on a traffic light is green. Yet in Japan and Korea, the green light is called the “blue light” (in Japanese, ao-shingō, 靑信号). This is not because of color blindness, and the lights are not actually blue. It is because languages divide up color differently.

The Japanese word ao (青) originally referred to both blue and green. The oldest color words in Japanese are said to be four: red, black, white and this ao.^[14] So green grass was ao, and a blue sky was also ao. When modern signals first arrived in Japan in 1930, the law described the color as “green,” but people called it the “ao signal,” and the name stuck. In 1947, the law itself came to call it “blue.”^[14] In 1973, to reduce the mismatch between name and color, the government decided that the green used should be, as far as possible, a bluish green.^[14] In effect, they adjusted the color to fit the name.

The Korean word pureuda is similar. Dictionaries define it as a bright, vivid color like a clear sky, deep sea or grass, a word that groups sky and grass together. The National Institute of Korean Language advises that in everyday use both “blue light” (paranbul) and “green light” (chorokbul) are acceptable standard forms.^[15] The Chinese qing (靑) also covers both green grass and blue sky. In China, however, the traffic light is called the lüdeng (綠燈), literally the “green light.” East Asian languages have generally not drawn a sharp boundary between blue and green, and traces of that remain in the names of traffic lights in Japan and Korea. Color is a matter of wavelengths the eye can see, but where to draw the line is a matter that language decides.

From Hanging an Arm Out to Blinking Light: The Turn Signal

Once intersection signals were settled, the car itself had to announce its intentions. At first, drivers stuck a hand out of the window to signal direction. An extended hand meant a turn toward that side, and a lowered arm meant stopping. But at night the arm was hard to see, in the rain the window had to be opened, and to the driver behind the arm was often not visible at all. The faster cars went, the more urgently a “visible signal” was needed, and the first answer was an arm-shaped signal borrowed from the railway. And for a long time afterward, turn signals were not standard equipment on every car.

A famous story says that around 1914 to 1915 the silent film actress Florence Lawrence devised a “signaling arm” mounted on the back of a car. When a button was pressed, a small flag on the bumper rose to show the direction, and when the brake was pressed, a “STOP” sign went up. But she never took out a patent, so there is almost no trace in the official record, and most of the story depends on later biographies and articles.^[16] What actually spread first was the European “trafficator.” It was a device in which an arm-shaped bar swung out from a pillar on the side of the car body to point the direction, like a railway semaphore shrunk to car size.^[17][17] As with traffic lights, the turn signal inherited an idea born on the railway.

Patents for electric turn signals came early. Edgar A. Walz Jr. of New York filed for a lamp signal with painted arrows in 1914 and received his patent in 1915, and Oscar J. Simler of Ohio, who filed in 1926, was granted a patent in 1929 for a combined turn, slow and stop signal.^[18] Neither design blinked, and automakers showed no interest. The blinking turn signal began to catch on only when Buick in the United States introduced it on a production car in 1939 under the name “Flash-Way.”^[18] A blinking light stands out more than a steady one, and the clicking sound meant that a driver who forgot to turn it off would notice on his own. After that, countries made turn signals mandatory by law, and hand signals retreated to a backup for when the lights failed.

The Color That Never Got Standardized

So is the turn signal the same color everywhere in the world? No. The UNECE (United Nations Economic Commission for Europe) regulations followed by Europe and most other countries specify amber for turn signals, and the rear is no exception.^[19] The United States, on the other hand, allows either amber or red for rear turn signals under its Federal Motor Vehicle Safety Standard 108 (FMVSS 108).^[19][20] That is why on many cars sold in the United States the rear turn signal blinks in the same red as the brake light, and the same model switches to amber in Europe.

The U.S. National Highway Traffic Safety Administration (NHTSA) has studied whether this difference is actually dangerous. A 2009 report (DOT HS 811 115) analyzed vehicle models whose rear turn signals changed from red to amber and estimated about 5.3% fewer two-vehicle crashes in which a car was struck from behind while turning, changing lanes, merging, or entering or leaving a parking space. An earlier 2008 report prepared for NHTSA by the University of Michigan Transportation Research Institute (DOT HS 811 037) had put the crash reduction for amber turn signals at about 22%.^[20] The estimates vary widely with method, but the findings are often cited as evidence that failing to distinguish brake lights from turn signals by color can cause confusion.

For English-speaking readers, this story is close to home. The first road signal was designed in London by a railway engineer, while the electric three-color light took shape in American cities such as Cleveland and Detroit, so the traffic light is something of a transatlantic hand-me-down. The two countries still read it differently. In Britain, red and amber lit together warn that green is about to appear, though drivers must still wait. American rules forbid showing red and yellow together, so the sequence goes straight from red to green.^[21] Britain also follows the UNECE rule of amber turn signals at the rear, so the blinkers on the car ahead are always amber, whereas on many American cars they are red.^[19]

Traffic lights began with the railway’s color rules, and turn signals began with the railway’s arm signals. Light travels farther than language, but how to read that light has always been decided by each society. That the boundary between green and blue differs from language to language, and the color of the rear lights differs from continent to continent, is less a sign of incomplete standardization than a trace of how each agreement was made.

Previous: Part 1: Dividing the Road