You see (and hopefully obey!) traffic lights on a daily or near-daily basis. But have you ever wondered why red means stop, green means go, and yellow means proceed with caution if you can’t stop safely?
Here’s an in-depth look at why traffic lights are colored the way they are.
A Brief History of the Tricolor Traffic Light
Before there were traffic lights, there were manually controlled traffic signals. The first documented one debuted in London’s Parliament Square in 1868. It included two lever-operated signs that could be illuminated with a gas lamp to ensure nighttime visibility.
These lights were usually controlled by police officers, so it makes sense that the world’s first electric traffic light was developed by Lester Wire, an American police officer, in 1912. This light, like almost all other traffic signals of the time, only had two signals: red for stop and green for go. A buzzer would sound right before changes to warn drivers.
In 1920, the world saw the first traffic light that both (1) had red, yellow, and green lights and (2) was three-sided or four-sided (so it could be used to control an intersection). Like its predecessor, this one was also designed by a police officer — William Potts of Detroit. Thanks to the inclusion of the yellow light, drivers didn’t have to hear a jarring buzzer each time the signal went from green to red or vice versa!
Of course, over the past 100+ years, the traffic light has undergone countless new innovations. But one thing has remained constant: the familiar stack of red, yellow, and green lights.
Wavelengths Matter for Visibility
Seeing (and obeying) a traffic light can make the difference between arriving at your destination without incident and getting in a life-threatening accident. So it makes sense that traffic engineers put an incredible amount of thought into the colors used for traffic signals.
There are several different factors that led to traffic lights looking like they do today. But one of these is more scientific than the others — it has to do with which wavelengths of light we see first.
As you can see above, the spectrum of visible light can be split into different colors. Each color has a corresponding wavelength, and red light has the longest wavelength of all.
The human eye can see longer wavelengths over greater distances. To understand why that is, you need to understand the phenomenon of “scattering” light. When visible light shines through the atmosphere, it runs into water droplets, pollution, and various types of other molecules.
Those molecules “scatter” light, meaning they absorb it and then radiate it in different directions. This process effectively amplifies the color of the wavelengths that scatter the most.
Shorter wavelengths of light scatter more easily. Blue light’s relatively short wavelength scatters especially easily — enough so that the sky appears to be blue in color.
Here’s a (very simplified) illustration of light scattering:
But if visible light runs into more interference from the atmosphere — like extra pollution — blue light and similarly short wavelengths scatter before we even see them. Longer wavelengths scatter last.
This is why sunsets tend to look red, orange, or yellow. As the sun sets, it has to travel through more of the atmosphere before it reaches us. Going through more of the atmosphere means it runs into more light-scattering molecules, so the shorter wavelengths scatter off long before the light enters our field of vision.
But what does that have to do with traffic lights? Because red light scatters last, red is the color we can see from the furthest away (and see most easily in rain, fog, etc.). The signal for “stop” is arguably the most important one for motorists to see, so it makes sense for this one to be red.
You might already know that red isn’t the easiest color for humans to see — a shade of yellowish green is actually the color our eyes distinguish most easily. So why isn’t this the color that tells us to stop?
Ultimately, traffic engineers determined that long-distance visibility was more important than a color’s overall noticeability. After all, drivers are looking for stoplights on the road, so lights don’t have to be extremely attention-grabbing. And when you need to bring a massive machine to a stop, having as much notice as possible is a great idea!
The light for “caution” or “slow down” is the next most important light for drivers to notice. But as you might recall, orange light scatters right before red. That makes it the second most visible wavelength, so why not use it instead of the typical yellow or amber?
The answer to this question is simple: red and orange lights are so similar in color that drivers would be likely to have trouble telling them apart. Yellow is the next-longest wavelength after orange, so it still travels pretty far. Green is the next wavelength down, and it’s easy to distinguish from the other two.
Traditional Associations With Each Traffic Light Color
Selecting colors with some of the most visible wavelengths is obviously important. But so is choosing colors whose historical associations match up with what they’re telling drivers on the road. Here’s a little about the traditional associations with each traffic light color (and why those associations make each one a good choice for use as a traffic signal).
Red: A Color of Danger
In many parts of the world, red is a color that’s loaded with emotion. It’s associated with danger and lust, anger and courage. Some studies even suggest that it can stimulate appetite.
The color psychology of red is complicated, and some of its historical meanings even seem to contradict one another. But even though this color’s associated meanings can vary by culture, humans as a species might have evolved to pay attention to red.
After all, it’s one of the most common shades found in animals with aposematic coloration — bright or otherwise noticeable coloration that signals the animal isn’t worth eating. The picture above shows a red-headed poison dart frog. He may be tiny, but his bright red head warns predators that they might get sick if they eat him!
Yellow: A Shade That Commands Attention
Yellow is bright and reflects an incredible amount of light. Because it’s so noticeable, it’s often used for warning labels (and road signs warning drivers of pedestrians, falling rocks, etc.)
But did you know that stop signs actually used to be yellow? That began in 1922 when the American Association of State Highway Officials convened to create an official stop sign design.
At this point, the electric traffic light had already been invented, and a red light already meant “stop.” So the Association naturally wanted to make stop signs red as well.
The problem? At the time, every available red dye was prone to fading relatively quickly. To ensure stop signs stayed visible for years, the officials settled on a yellow design. But once porcelain enamel was developed in the 1950s, the stop sign became the red octagon we know today.
Green: A Carryover From Railway Signals
Green doesn’t have quite the evolutionary significance of red — outside of railway signals and traffic lights, it hasn’t historically been associated with safety.
However, green looks very different from red and yellow, and because it had worked so well as a railway signal, it was used for traffic lights as well.
Railroad Signals Inspired the Colors of Traffic Lights
The very first railroads in Britain were developed in the 1820s. A couple of decades later, most of them adopted a system of both colored lights and semaphore signals.
The signals looked something like this:
This system was a complex one that evolved over time, but for our purposes, the colors of the lights (and their meanings) are of the most interest.
Railways used three colors of light to signal to oncoming trains. Red meant “danger” (or stop), white meant “safety” (or go), and green meant “proceed with caution” (much like a yellow light does today).
This system was better than having no lights at all, but it did come with some issues. Red and green lights were easy enough to see and understand, but it was also easy for engineers to see an especially bright star in the night sky (or even just an ordinary white light) and think it was safe to proceed when it was not.
There are a handful of sources that claim a grisly railroad accident was, in part, responsible for removing white from the lineup. A red railroad light allegedly lost its red cap, meaning it only gave off white light. Thinking it was safe to proceed, an engineer accidentally plowed into another train.
While this story is certainly plausible, there isn’t much evidence to support it, and different accounts have conflicting details. Nonetheless, it eventually became clear that the red-white-green system wasn’t quite working, so yellow (or amber) eventually replaced white as a broader effort to enhance safety.
Are Red, Yellow, and Green the Standard Colors Everywhere?
In many, if not most, parts of the world, you’ll find that traffic lights are red, yellow, and green. That’s largely due to the Vienna Convention on Road Signs and Signals. This United Nations treaty was created in 1968 to establish consistency in traffic lights and other road signals across international borders.
The treaty has been adopted in more than 70 countries. However, many countries that have not signed or ratified the treaty follow at least some of its guidelines. For example, the United States hasn’t signed or ratified, but it still uses red/yellow/green traffic lights.
However, some countries use different-colored lights. For example, in Japan, the light for “go” is often blue instead of green.
The reason why comes down to the history of the Japanese language. For centuries, the Japanese only had four words to describe different colors: red, white, blue, and black. The word for blue, “ao,” was also the word used to describe something green.
It wasn’t until the 1950s that Japanese schools started teaching kids separate words for “green” and “blue.” “Midori,” a word that technically described a very greenish blue, started to be used for the color English speakers would just call “green.”
Of course, this type of linguistic change isn’t something that just happens overnight. Practically speaking, many Japanese citizens used (and still use) “ao” to denote both blue and green.
In official government documentation, Japanese traffic lights are described as “ao,” not “midori.” And even though Japan never signed the Vienna Convention on Road Signs and Signals, the government wanted to ensure that traffic signals were familiar enough for foreigners to navigate easily.
So they could still describe the lights as “ao” while simultaneously ensuring they were green enough for drivers from other countries to understand, the Japanese government struck an interesting balance.
In 1973, the government officially ordered that all the country’s traffic lights should be green — they just had to be the bluest possible shade of green. But as the above picture shows you, some of the lights in the country appear to be a shade of bright, sky-like blue.
What if a Driver Has Color Vision Deficiency?
As the above graphic shows you, there are several different types of color blindness. Depending on which type a person has, it might prove to be difficult or impossible to tell the difference between red and green. So, as you might imagine, color vision deficiency can make driving difficult (and sometimes even dangerous).
Fortunately, many countries have their traffic lights structured in such a way that color-blind drivers can understand a light’s meaning based on its position.
In most places with vertical traffic lights, the lights are always in a column with red on top, yellow in the middle, and green on the bottom. Some countries and even some U.S. states have horizontal lights. With this design, the red light is on the left, the yellow is in the middle, and the green is on the right — at least in most areas.
In Japan and some other nations, most horizontal traffic lights have the red light on the right and the green on the left. You might wonder why light placement varies by country, but it all comes down to the way a particular nation reads a page.
Traffic engineers want people to notice red lights as soon as possible. As a result, these lights are placed where a given culture starts to read a page. In countries that read from left to right, the red light will be at the top of a vertical traffic light and at the left of a horizontal one — after all, these cultures read a page starting at the top left of a page.
Japanese is read from right to left. As a result, the majority of Japanese traffic lights have the red light at the top of a vertical light and to the right of a horizontal light.
Most countries allow people with color vision deficiencies to drive. But some — including most countries in the Association of Southeast Asian Nations (ASEAN) — prohibit people with any kind of color blindness from driving.
Fortunately, engineers have come up with traffic lights that use shapes as well as colors. These lights let color-blind drivers rely on distinguishing shapes rather than colors, and they also improve reaction time in drivers with normal color vision.
Gain a Better Understanding of the World — One Traffic Light At a Time
Traffic lights help keep us safe on the road, but that doesn’t mean it’s not annoying to be stuck at a red light! The next time you’re waiting at an intersection, take a moment to appreciate the years of history and innovation behind the humble traffic light.