- Thank you to Squarespace for supporting PBS. - I've always wondered why are stoplights red? Searching the web or asking an AI, turns up a frequent answer. Red light with a longer wavelength is scattered less by the atmosphere, making it visible from farther away through fog or rain. This is a convincing answer. It agrees with a real scientific phenomenon. Red light does have the longest wavelength in the visible spectrum, and red light is scattered less by the atmosphere than blue light allowing it to penetrate farther.

That's why the sky is blue and why sunsets are red. Unfortunately, when it comes to traffic lights, this answer is wrong. Physics is not why traffic lights look like this. Today we're gonna illuminate the real story of why red means stop and why green means go. It's a tale that involves deadly disasters, exploding lamps, forgotten inventors, the neuroscience of vision itself, and even some 35 million year old fruit.

This is how the humble stoplight ended up being nearly scientifically perfect for its job, despite nobody understanding that science when we invented it. So on your marks, get set, that (bright music) Hey, smart people, Joe here. The humble traffic light is a device so ubiquitous, it's often overlooked. - Hey, you, why don't you pay attention while you're going? - Well, almost overlooked because its very purpose is to make us look and to communicate unambiguously in the name of safety.

I mean, imagine a world without this trichromatic wonder. Busy streets tangled in chaos as vehicles and pedestrians battle for the right of way, utter unimaginable anarchy. This system of lights is, in my view a true marvel of design and safety, one that seems almost perfectly crafted to exploit human psychology and attention. So why does green mean go and red means stop. We'll get to that. But first we have a misconception to debunk. Rayleigh scattering is a physics phenomenon where red light is scattered by molecules in the atmosphere less than blue light. So it's true that red light having the longest visible wavelength

around 700 nanometers or so, allows it to penetrate the atmosphere farther than other colors, scattering nearly 10 times less than blue light. So it should be easier to see red in rainier foggy conditions, right? Well, not really. Here's the thing. Rail scattering doesn't really happen in fog or rain. In Rayleigh scattering, light is interacting with particles many times smaller than the wavelength. We're talking less than a billionth of a meter, stuff like air molecules. But fog droplets are huge compared to light, thousands of times bigger than visible light wavelengths. At that scale, a completely different type of scattering happens, Mie scattering,

and it basically scatters all visible light by the same amount. This is actually why fog and clouds appear white, not colored, because they're scattering every color of light. So that whole red penetrates fog better thing is a complete physics misconception. Now weirdly, the light receptor cells in our eyes are most sensitive to green light. So for two colored lights with the same power, the green one will always appear brighter. So if we were designing this based on our sensitivity to light, you'd think maybe we would've picked green for the all important stop signal. Yet we didn't. So if physics or light sensitivity aren't the reasons we picked red for stop, what is the reason?

Well, the answer involves a few historical accidents, like literal ones. Hey, you guys like trains. (train horn whistles) In the earliest days of the railroads, there were no standard rules for signaling things like the track in front of you is clear. The first signals were given by actual people standing next to the track. Those evolved into flags or even a ball that would raise up when it was safe to proceed. Sometimes that was red, but it was often white too. The term high ball even became a kind of slang term, meaning good to go.

Gen Z can bring that back. If you wanna flex some new fire lingo to slay over in the train chat. Okay. Rude. By the late 1800s railroads in the UK were using a system of signals with moving arms with the addition of colored lights for use at night or in low visibility. Now, in this early system, a red light was the standard stop signal, but the signal for go was just plain white and green. Not yellow was commonly used as the caution signal, but to change the white go to stop the arm would move a piece of red glass in front of a lamp. But in 1876 during a snow storm in England, a frozen signal arm failed to move into the stop position,

leaving the white all clear, light, visible, and leading to a deadly (indistinct). After the crash, investigators recommended that instead of white, a clearly identifiable color be used to signal all clear ahead. Now, since green glass was common and easy to produce, that became the new signal for go. Green means go was literally an accident. But different glass makers use different formulations for glass, resulting in sometimes dozens of different shades of green and red being used. Reds could range from crimson to orange. While one railway is green might be close to yellow, and the other's closer to blue.

This was obviously a recipe for confusion. In 1899, a manager at the Corning Glass Company attended a lecture on colorblindness, which often manifests as the inability to distinguish green and red, and he instantly recognized the need for standardized glass colors for signals. So Corning established their own optical laboratory to test the best recipes and colors of glass that would be clearly visible as red or green at long distances. And in 1908 standardized signal colors were first established. Now all of that happened before cars were even a thing. Of course, by the mid 1800s the streets of big cities like London were crowded with traffic from horse-drawn vehicles leading to busy and dangerous intersections.

And most of these policemen would direct traffic by hand or people just work it out for themselves. But if you happen to find yourself in London today, admiring the majesty of Big Ben, look above this doorway where you will find a plaque dedicated to railway engineer John Peak Knight, inventor of the world's first traffic light. In 1868, Knight erected a 22 foot cast iron pillar at this intersection. And during the day, a policeman would control little flags or signs on the pillar to control traffic. But at night, gas lamps could be switched between red and green, likely adopting the standard use at the time of green meaning proceed with caution and not meaning go.

Now, the intersection was safer for about a month because on January 2nd, 1869, flammable gas accumulated in the hollow pillar and it explodes and severely injured the policeman operating it. Come on though, it's just one little explosion. Right? Yet thanks to this accident, the city of London rips the thing out and doesn't try traffic lights again for 60 years until 1929. And by that time, America had become the car crazy nation that we still are today. Model Ts and Roadsters roared through the teens and into the '20's and quite often into each other, America needed traffic signals and three separate people would lay claim to the invention.

Meet Lester Farnsworth Wire, Salt Lake City police officer. In 1912 taking inspiration from Corning's standardized railway signals. The very appropriately named Wire, builds the first electric traffic light by hand painting red and green bulbs and mounting them in a yellow wooden box. Locals called it Wire's pigeon house, and they mostly ignored it. Well Wire enlists in World War I. And while he's over in Europe, his invention begins to gain popularity. But when he gets back from the war, he never patents the device.

Earns basically nothing from an invention that changed the world. In 1920, William Potts a Detroit police officer. He adds the crucial yellow light Again, taking inspiration from railroads where amber lights had become the standard signals for caution by that time. The older two color signals had caused numerous crashes thanks to green just changing straight to red with no warning. Pott's first three color signal was built for just $37 worth of wire and electrical parts. It's like a measly $600 in today's money.

Unfortunately, as a city employee, Pott's was prohibited from patenting his invention and he too died in relative obscurity. And finally, in this tiny forgettable 1935 obituary that I located, after nearly a full hour of internet sleuthing, we bid goodbye to a man named Hugo Kleinsteuber, who 1921 automated the traffic light signal so that it could be operated without human intervention. Another tragic ending for a man whose bright idea is so taken for granted We curse at it when we're running late for our Chipotle to go order. So we've established these colors weren't chosen by physics, they were chosen by accidents.

But it turns out there is a scientific reason why red and green are exactly the right colors for this job, and that's way older than traffic lights. So red and green traffic lights as we know them today, were born on the railroad and adapted to city streets, thanks to a few handy policemen about a century ago. But red and green do turn out to be, well, basically the best colors you could pick for traffic signals. And that's because we have a specially tuned ability to see red and green. And that is much older, about 35 million years older. You see, back then in the old world of monkeys and apes, including the distant ancestors of humans, only had two types of color sensitive cells in their eyes.

One of those was most sensitive to bluish light, and another most sensitive to greenish yellow. But back then, the bit of DNA encoding that green sensitive receptor was duplicated and that new second copy mutated until it became a little more sensitive to the red end of the spectrum, and before this, primates couldn't really tell red and green apart, red and green both stimulated the same receptor. So the brain couldn't tell the difference between dim green and bright red. But with this new third slightly redshifted color sensitive cell, red and green light would stimulate these at different levels, eventually allowing the brain to tell the two colors apart.

Today, all primates who originated in Africa, including us, have these three color receptors. But why did this adaptation spread so widely? What was so important about seeing red versus green, but one possible answer is fruit. Being able to differentiate between red and green may be the optimal setup for detecting ripe fruit against green leaves. But that's not the only hypothesis out there. Perhaps this red green vision also let our ancestors detect young red leaves, which are often more nutritious than crusty old green ones.

It may also have aided in detecting predators in camouflaged environments, or even as aided to sexual signals, you know, blushing and all that. Anyway, the point is being able to discriminate red from green helped our ancestors survive better. So tricolor vision became more common. And interestingly to this day, monkeys that evolved outside of Africa almost exclusively still have that ancient two color receptor system. Kinda makes you wonder what the rainforest looks like to them. So if we evolved red green vision to find fruit or leaves 35 million years ago, why do we tend to associate red with danger today?

I mean, culturally speaking, red seems almost universally associated with alarm. Red flags, stop signs, fire trucks, you know, those spinning red lights when somebody breaks into a secret base, it's often claimed in color psychology, which I guess is a real thing that red is innately associated with danger and green with calm. I mean, red's the color of blood, anger. Green, on the other hand, is supposedly peaceful, like a nice pleasant walk in the park, ah, But as is all too often the case in psychology, early studies that suggested these supposedly universal links, they couldn't be replicated by other people.

And color is far from culturally universal. In India, red's often associated with purity. And in China it stands for prosperity. And in nature, venomous or toxic animals, they use a whole rainbow of colors to send those, you know, do not eat me danger signals. So it seems that we were programmed by evolution to be attracted to red, not to view it as a danger sign. It's a color that we are uniquely primed to notice, just maybe not for the reasons that people might have thought back when they were designing railroad signals in the 19th century. And that's a good thing because when it comes to these, noticing is really what it's all about.

Okay, listen, we've all been there. Okay, you're driving home daydreaming about something, maybe your next YouTube video topic, I don't know. And next thing you know, you realize you haven't been consciously paying attention to the road for the past few miles yet somehow your subconscious mind was still able to process what was happening and control the vehicle even stop at red lights. It's a phenomenon called a highway hypnosis. Luckily for you and me, we have one more special feature of our visual system that makes green and red ideal for getting our attention when we aren't paying attention.

In the late 1800s, a few scientists began to notice that certain combinations of colors seemed impossible. We can easily imagine a combination of blue and red as purple, but we can't perceive a color that's simultaneously blue and yellow or reddish green. This is because our eyes and brain do an interesting little trick. In our visual system a stronger red signal suppresses the green while a stronger yellow signal suppresses blue. These pairs of colors are treated as opposites by the literal circuitry of our visual system. Now this system ends up being faster and more reliable than trying to sense every color individually.

You know, instead of asking what color is this exactly, your brain asks, is this red or green? Is this blue or yellow? Having quick and reliable color vision matters when you need to react fast. Like when you're moving inside of a potentially deadly steel contraption weighing several thousand pounds and the lights just changed. When scientists tested simple reaction times, pressing a button when you see a colored light, people responded fastest to red with yellow in second place, then green, and finally slow old blue. Of course, that wasn't published until a century after traffic light colors had already been settled. But somehow those train guys and policemen

stumbled on the exact colors that we react to the fastest. Of course, that doesn't mean that traffic lights are perfect. I mean, just ask the eight or so percent of males who are red/green colorblind. Research has found that they respond significantly slower to red lights and yellow lights compared to people with normal color vision. Now, there are some interesting solutions out there. Parts of Canada use signals with distinct shapes added so colorblind drivers can use position and shape, but most of the rest of the world seems to be stuck with this.

'Cause once everyday technologies like that become widely accepted, the simplest things can often be the hardest to change. Here's what I keep thinking about. (gentle music) We like to tell ourselves scientific explanations about things that well weren't always scientifically chosen. The red penetrates fog better story, but sounds authoritative. It invokes real physics. It feels rational, but it's a classic example of a just so story.

An explanation invented after the fact to make a historical accident sound like something that was intentionally designed. It strikes me as a good lesson for science, and we may want to take something that works and reverse engineer a neat explanation for why it must be that way. When really sometimes it's just an accident that led us to the right answer. Thanks to luck, which in many countries is associated with green, unless you're in China where luck is red, that's ironic. But at the same time, something that seems accidental may end up having real legitimate science behind it,

even if the path to that ideal solution was totally unintentional. So the next time someone tells you red lights are red because of wavelengths and fog or whatever, you tell them the real story. It was a happy accident involving ancient fruit, trains, an exploding lamp, little birdhouse in Utah, and this, that story isn't as tidy, it's full of accidents, but this ended up preventing quite a few accidents of its own by getting us to the right answer the wrong way. Stay curious. - [Announcer] Thank you to Squarespace for supporting PBS.

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