Red means Stop, Green Means Go!

I was reading “Go, Dog. Go!” (which I would recommend to any parent) to my two-year-old daughter when we reached the page where a bird is crossing the street and a barrage of dogs driving cars are barreling down the road to get to a party in a tree. The bird yells, “Red means Stop, Green means Go.” She then looked at me with her inquisitive eyes and asked, “why is green go and red stop?” Not knowing the answer, I had to save face, so I told her, “Mommy was going to sing her some songs.” I was saved by misdirection. But it got me thinking — why is green meant for go and red meant for stop? This led me down a rabbit hole, which led to some fascinating discoveries and people along the way.

When attacking this I was not sure what I was going to discover, but as with anything, you find yourself going down rabbit holes and trying not to get sidetracked. I started learning that red has always been associated with danger — e.g. fire, blood, the Romans, the devil, the red zone in football when your team is already losing. It also turns out red has the longest wavelength of all the colors, which lets it be seen from great distances, makes it best for penetrating situations such as fog or dust, and, to top it off, makes it one of the softest lights. Green, on the other hand, has a shorter wavelength and is more pleasant, or easier on the eye, and is associated with a more calming influence in life: nature, hope, renewal, St. Patty’s Day — that one I just threw in. But why red and green? Why not orange and blue, or two other colors? I carried on with my research and found out that the first traffic light color system originated from the navigational lights of old wooden ships. They used the lights for marking entrances and boundaries for ports and harbors. Red glass was used for making red lanterns — not because it could be seen from great distances, or because it was best for penetrating fog (science only proved that coincidentally, years later), but thanks to the abundance of port and wine bottles that were readily available and readily consumed at the time, thanks to a love of alcohol. Green glass was also used to make lanterns, but because light-colored glass exposes its contents to sunlight far more than dark or red glass would — altering the liquid’s integrity, longevity, and taste — there was not as much green glass around. This gradually evolved to identify red with port and green with starboard. The fact that alcohol not only decided but created our traffic directions is another just-perfect bit of irony.

Naturally, as time progressed and technology flourished, factory machines took up this color-coded system, using red to indicate that the machines were off and green that they were running. The railroads took inspiration from this and soon adopted the color-coded signal system too, but took it a step further: red for danger, green for proceed-with-caution, and then a third color, white, to communicate safety, or all is good. At the time, colored bulbs were not around yet, so colored lenses were used in their place. Over time, however, those lenses would weaken and fall out of place. There was a particular case when the red lens dislodged from its socket, and what remained was a brilliantly beaming white light — communicating “all is safe” to an oncoming train. This caused a massive incident, two trains in a catastrophic collision, and because of it the white was replaced with yellow, which brings us to the modern-day traffic light.

As with most firsts, there is debate over who was the very first, who made it better, and who capitalized on it. My findings turn up four different men, all contributing in different ways to the traffic light. J.P. Knight, from London in 1868, can be credited with creating the very first traffic light, using a semaphore — an apparatus for communicating signals over long distances. However, this system was not a success; it lasted only a few years, largely because it killed the police officers operating it in an explosion. Years later, in 1912, Lester Wire, a policeman in Salt Lake City, can be credited with the first actual electric traffic light, using red and green lights. His invention took inspiration from the railroad lights, just as the railroads did from the factories before them, and the factories from the ships.

Prior to his invention, police officers would stand in the middle of busy intersections on small platforms slightly higher than a kitchen ladder, directing two-way, if not four-way, traffic. You have to remember, driving a car at this time was relatively new: there were no laws, no rules, no lanes, no crosswalks, no driving exams — it was pure chaos. Think GTA Vice City, or San Andreas for those of you who know the game. You could do whatever you wanted, wherever you wanted, however you wanted. It has been said that pedestrians in the street were “fair game” if they did not move out of the way fast enough. Pretty much modern-day LA.

Fast forward eight years and some 1,650 miles, and we arrive in Detroit, where officer William Potts created his three-color, four-way traffic signal. The addition of the amber light was finally adopted, and it was for caution — but it was the positioning of it that really made all the difference. Having the light placed in between the red and green communicated to drivers to slow down, as the green light preceded it and the red was soon to follow. By the mid-1930s, the three-color signal was the ruling standard for all lights, and it is still being used today, all over the world.

We now come to Garrett A. Morgan, the last of the four musketeers; he might have been late to the game, but he had an ace up his sleeve. Garrett was already a well-established inventor at the time, having created hair products and the first proven gas mask — a device so successful that the US Government used the blueprints in WWI. His vision started as he was walking down the street and witnessed a horrific accident between a horse-drawn carriage and a car at a problematic intersection in Cleveland; after observing what I can only imagine as absolute carnage, he invented his traffic signal. It was powered by batteries, or by electricity if available from overhead wires. Much like Potts before him, his too featured three positions, but slightly different: the three positions were Stop, Go, and an all-directional Stop. That third position paused traffic in all directions before allowing vehicles to proceed on either of the intersection’s perpendicular roads. This was a great feature; not only did it enhance safety for motorists at intersections, it also made pedestrian crossings safer, as no cars were allowed to move. In addition to the “all stop,” he also incorporated noise. Using bells to signal that the traffic signal was changing directions was brilliant — it allowed pedestrians to cross roads with much more confidence and safety. Martin Knopp, associate administrator for the Federal Highway Administration’s Office of Operations, is quoted as saying, “Consideration of all modes of travel and improving safety for everyone, especially with an emphasis on vulnerable road users, continues in today’s development of traffic control devices,” about Morgan’s traffic signal. The ace up Morgan’s sleeve was that he was the first to apply for and acquire not just a US patent, but Great Britain and Canadian patents as well — giving him the ability and the upper hand to use and sell his traffic signal both nationally and internationally. His traffic management system was used so prominently throughout America that GE purchased it for $40,000 ($709,000 adjusted). It was not until Potts’ tricolor lights came about that those would eventually take over.

Today, we still rely on tri-colored lights, sounds, and timers, but traffic signal systems have become much more advanced. Inductive loops now detect waiting vehicles by measuring the metal in cars; moving magnets create an electric current as vehicles pass over, signaling the system that a car is waiting. The traffic lights themselves have improved as well—they are now larger, equipped to be visible in any lighting condition, and use energy-efficient LED bulbs. Many lights also have sensors to detect the strobing lights of EMS, police, and fire trucks, automatically granting them the right of way. However, these advancements come with a cost. A typical traffic light setup ranges from $250,000 to $500,000, with annual maintenance around $8,000. As a result, some cities and towns explore alternatives like stop signs, yield signs, and traffic circles.

Let’s start with what’s happening under your tires. You have probably heard that traffic lights have “weight sensors” buried in the road — that the signal is patiently waiting for a car heavy enough to trip it. It’s a great story. It’s also wrong. What’s actually cut into the pavement is an inductive loop: a coil of wire with a small current humming through it. When a big slab of metal — your car — settles on top, it warps the loop’s magnetic field and the system goes, ah, somebody’s here. It isn’t weighing you. It’s a metal detector, the same gadget that beeps at you at the airport, scaled up and paved over. (It’s also why a motorcycle or a carbon-frame bicycle can sit at a red light until the end of time — not enough metal to convince the loop it exists.) Newer intersections have skipped the buried wire altogether and simply watch the road with radar, microwave, and video cameras that count cars, measure the gaps between them, and rewrite the timing on the fly. The light isn’t running a stopwatch anymore. It’s paying attention.

Then there’s the trick every kid in the back seat notices and no adult can quite explain: how does the fire truck catch every single green light? That isn’t luck, and the driver isn’t flipping switches. It’s called signal preemption, and the most common flavor — look for the little white strobe or sensor riding on the signal arm — goes by the name Opticom. The emergency vehicle throws an invisible infrared pulse, flickering at an exact rate like strobe-light Morse code, at a detector on the signal. The intersection reads the code, drops whatever it was doing, and hands that direction a green before the truck even arrives, while a small confirmation light tells the driver it worked. Fire engines and ambulances get the platinum treatment — red becomes green, right now. Buses get a gentler version called signal priority: the light won’t flip for them, but it’ll stretch a green a few extra seconds or trim a red to keep them on schedule. Newer setups have traded the strobe for GPS and the cloud, but the bargain is the same — certain vehicles get to ask the intersection for a favor, and the intersection says yes.

Now look down at the pole beside you — at that button you’ve stabbed nineteen times waiting to cross. Does it actually do anything, or is it just there to give your thumb a job? The honest answer is: it depends, and the truth is far better than the myth. Think of it less like a light switch and more like the call button on an elevator. You aren’t flipping the walk signal on; you’re placing a reservation in a very busy system that will get to you when the traffic pattern allows. At a sleepy intersection with light foot traffic, that button is doing real work — no press, no walk, because there’s no sense stopping traffic for a sidewalk nobody’s standing on. In a dense downtown, the walk phase is already baked into every cycle whether you touch it or not, so your press changes precisely nothing. And here’s the detail that earned these buttons their reputation: when cities computerized their signal timing, a lot of the older buttons quietly went dead, but prying them off the poles cost money, so they stayed. New York famously left something like three out of four of its pedestrian buttons wired to nothing at all. London hands you a reassuring “WAIT” light that has no say in when the green man actually appears. Psychologists even have a term for the comfort a dead button provides — the “illusion of control.” Pressing something, anything, makes the waiting feel less helpless. So sometimes you’re casting a vote, and sometimes you’re petting a placebo. You will rarely know which.

But before you write the button off, press it once for someone who truly needs it. Even a button that won’t hurry the light is very often still wired to the part that matters most — the part that helps a blind pedestrian cross. Modern crossings carry what’s called an Accessible Pedestrian Signal, and it is a quiet little marvel. It ticks out a steady locator tone so a person who can’t see can find the pole and the button in the first place. When it’s safe to walk, it announces it out loud — a rapid chirp, or an actual spoken “Walk sign is on to cross Main Street.” And the button itself usually wears a small arrow that physically vibrates during the walk phase, so someone who can neither see the signal nor hear the chirp can feel that it’s finally their turn. Garrett Morgan bolted bells onto his signal a century ago so people could cross by ear. We’ve just taught the bells to talk.

Which leaves one small mystery we’ve been cruising straight past: why are the lights stacked the way they are? Red on top, yellow in the middle, green on the bottom — always, everywhere, as if carved in stone. It nearly is. The order was standardized around 1930, and the logic behind it is quietly brilliant. Red rides up top because it’s the one you cannot afford to miss — highest in the air, hardest to hide behind a truck or a branch, visible from the farthest away. But the deeper reason is for the millions of people who are colorblind and genuinely cannot tell the red from the green. If the position never moves, the color doesn’t have to. A colorblind driver doesn’t need to see “red” — they just need to know that the lamp glowing at the top means stop and the one at the bottom means go. The order isn’t decoration. It’s a second, silent language for everyone the colors leave behind. (And on the occasional horizontal signal, the rule simply tips on its side: red always lives on the left.)

Here’s where it turns properly futuristic — and a surprising amount of it is already sitting in driveways. For a hundred years the conversation ran one direction: the light talked, you obeyed. That’s beginning to change. A growing number of cars now listen to the lights directly, through something called vehicle-to-infrastructure communication. The signal broadcasts its own state — which color it’s showing and exactly how many seconds remain until it flips — and the car reads the message. Audi has been doing this on public roads since 2016: in a handful of U.S. cities, certain Audis put a little countdown in the dash telling you how long that red has left, so you can stop white-knuckling the brake and finish your sentence. The same system can murmur a suggested speed — ease back to 31 and you’ll glide through the next three lights green — turning the red-light gauntlet into a smooth, fuel-sipping “green wave.” It’s the whole difference between guessing and knowing.

Now aim that idea at a car that drives itself. A self-driving car doesn’t want to merely see the light through a camera and hope it guessed right — it wants the intersection to tell it, in plain data, “green for four more seconds, then yellow.” Feed a car that and it can flow through a corridor barely brushing the brakes; feed thousands of those cars’ positions back to the intersection and the light itself gets smarter, retiming on the fly, even waving a whole platoon through with almost no stops. Some cities are already pilot-testing AI-run signals that learn their own traffic and adjust it minute by minute. The endgame is an intersection that behaves less like a dumb lamp on a timer and more like a calm air-traffic controller — talking to every car at once, choreographing the entire messy ballet. We began with a policeman on a wooden box waving his arms at the chaos. We may well end with no light to look at at all, because the car already knows.

Green and red have become more than stop and go on traffic lights; they have become so mundane, so common in our day-to-day lives, that we don’t acknowledge or realize how important, vital, and necessary they are — or how reliant we are on them. They are used in almost every aspect of our lives, all over the world, to convey safety and danger without the need for a translator. Paratroopers use them in a plane when prepping for a jump; they’re used for measuring oxygen pressure when scuba diving, on hospital gauges, fuel gauges, and needle gauges, in product sales growth, on traffic maps — anything you can think of. They have become the yin-and-yang tool for safety and information. Hell, Tom Cruise used a stick of bubble gum, half green and half red, yelling “Red Light, Green Light” before throwing it onto a helicopter at the end of Mission Impossible to save the day. These colors made the impossible mission possible — and saved Tom Cruise in the process.

Eager and waiting for my daughter to wake up, I was now fully ready to answer the question of why red and green are the lights we use. Once she was awake, I was planting seeds to get her full attention, hoping she would ask the question again now that I was prepared. I was asked nothing about colors. Crickets. Nothing. Zilch. It was not until my wife stepped in, saw my struggle, and gave her a strawberry and a green grape — then started playing stop and go with eating the fruit — that she finally asked me why red and green. Excited, but then a calmness came over me; realizing she was only two and had the attention span of a flea, I simply answered, “because those are pretty colors.” She agreed, adding some of her other favorites to the mix, and then threw her plate of fruit all over the floor, saying, “Green light, Daddy.”