Why are Traffic Lights red, yellow & green?

Summary

Who Smarted? Podcast Summary
Episode: "Why are Traffic Lights Red, Yellow & Green?"
Release Date: May 14, 2025

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Introduction

In this episode of "Who Smarted?", Trusty, an inquisitive young listener, finds himself waiting impatiently at a bustling intersection. Frustrated by the seemingly endless red light, Trusty strikes up a conversation with Trevor, the personified traffic light. This engaging dialogue sets the stage for an entertaining exploration into the history and science behind traffic lights.

The Origins of Traffic Lights

Trusty ([00:00]): "I'm just standing at the corner waiting to cross this busy street. ... But the light should turn green any second now."

Trevor ([00:30]): "No, not forever forever. It's actually on a timer, like all traffic lights."

Trusty’s curiosity leads him to question Trevor about the invention and functionality of traffic lights, particularly why they use the colors red, yellow, and green.

The First Traffic Light in London (1868)

Trevor ([03:28]): "The first traffic light was a far cry from what you see on the roads today. But it was a start."

Trevor explains that the very first traffic light was installed in London in 1868, right in front of the British Parliament buildings. At that time, horse-drawn carriages dominated the streets, leading to chaotic traffic and pedestrian congestion on Westminster Bridge. To manage the heavy traffic, the semaphore-based traffic light was introduced.

Semaphore System and Early Operations

Trusty ([05:17]): "Smarty pants. True or false? The first traffic light was powered by gas lamps and had to be manually controlled by a police officer. If you said true, you're right."

Trevor ([05:36]): "Semaphore is a signaling system in which a mechanical arm or flag extends from a pole. ... drivers needed to stop or go based on the arm's position."

The semaphore system involved mechanical arms and gas-powered lights, operated manually by a traffic cop. A horizontal arm signaled drivers to stop, while a 45-degree angle indicated it was safe to go. However, this system had its limitations, especially in low visibility conditions.

Transition to Electric Traffic Lights (1912)

Trevor ([09:24]): "The first electric traffic light was invented in the USA by a police officer named Lester Wei."

In 1912, the United States saw the advent of the first electric traffic light, significantly improving upon the semaphore system. This innovation marked a pivotal shift, making traffic management more efficient and reducing the reliance on manual control by police officers.

Introduction of the Yellow Light (1920)

Trusty ([13:22]): "So the first traffic light didn't have a yellow light? How did they warn drivers?"

Trevor ([14:13]): "Yes, the first traffic lights went straight from green to red. To address safety concerns, Officer William Potts introduced the yellow (amber) light in 1920."

Officer William Potts played a crucial role in enhancing traffic light functionality by adding the yellow light. This intermediate signal provided drivers with a warning to slow down before the light turned red, thereby improving safety at intersections.

Automation and the First Automatic Traffic Light (1922)

Trevor ([14:46]): "In 1922, the first automatic traffic light was installed in Houston, Texas. This system used built-in timers to control light changes."

The introduction of automatic traffic lights revolutionized traffic management by eliminating the need for manual operation. Houston's implementation of timer-based traffic lights allowed for more consistent and reliable regulation of traffic flow. This advancement also enabled the reallocation of police officers to other essential duties, saving significant municipal costs.

Modern Traffic Light Technology

Trevor ([15:30]): "Today's traffic lights do way more than just change colors. Sensors and automation help manage traffic flow effectively."

Modern traffic lights incorporate advanced technologies such as green waves and sensors:

• Green Waves ([15:40]): "A system where multiple traffic lights are coordinated to turn green in succession, allowing cars to pass through several intersections without stopping."

• Sensors and Induction Loops ([16:09]): "Digital sensors detect vehicles approaching an intersection, while induction loops embedded in the road sense the presence of vehicles to adjust light changes accordingly."

These innovations enhance traffic efficiency, reduce congestion, and improve overall safety on the roads.

Global Differences in Traffic Lights

Trusty ([16:47]): "Are all traffic lights the same worldwide, or are there differences?"

Trevor ([17:04]): "While many aspects are standardized for safety, some variations exist globally. For example, in Japan and Quebec, traffic lights are horizontal instead of vertical."

Key differences include:

• Orientation: Some countries use horizontal traffic lights, especially in regions where driving is on the left side of the road, like Japan.

• Color Ordering: In countries driving on the right (e.g., Canada), horizontal lights display red, amber, and green from left to right. Conversely, in left-driving countries, the order is reversed.

• Terminology: In South Africa, traffic lights are colloquially referred to as "robots."

Conclusion

By the end of the episode, Trusty gains a comprehensive understanding of the evolution of traffic lights, from their early semaphore origins to the sophisticated, automated systems in place today. The conversation underscores the importance of these devices in managing traffic flow, enhancing safety, and adapting to technological advancements worldwide.

Notable Quotes:

• Trevor ([03:28]): "The first traffic light was a far cry from what you see on the roads today. But it was a start."

• Trusty ([07:00]): "Smarty pants, any idea why those colors were used to represent stop and go?"

• Trevor ([15:30]): "Today's traffic lights do way more than just change colors. Sensors and automation help manage traffic flow effectively."

This episode of "Who Smarted?" successfully combines humor with insightful historical and scientific information, making the topic of traffic lights both educational and entertaining for young listeners and their families.

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Credits:
Written by Phil Greenlight Jeremy
Voiced by Mike, Adam, Tex Davis, and others
Produced by Adam Tex Davis and Jerry Colbert
Technical Direction and Sound Design by Josh
Theme Song by Brian Mixed Signals Suarez with lyrics by Adam Tex Davis

Produced by Atomic Entertainment.

Transcript

[Trusty]

And now it's time for Who Smarted? Psst. Hey, smarty pants. I'm just standing at the corner waiting to cross this busy street. Wow, it's a really busy street. And it feels like I've been waiting quite a while. But the light should turn green any second now. Any second. Like right now. Okay, it's still red. Maybe right now. Nope, still red. Oh boy, this is taking forever.

[Trevor]

No, not forever forever. It's actually on a timer, like all traffic lights.

[Trusty]

Who said that?

[Trevor]

Up here, I'm the one you're complaining about.

[Trusty]

Yes, a talking traffic light.

[Trevor]

But of course the name's Trevor and I run these streets.

[Trusty]

Gotcha. Also, why is it that everyone's name always starts with the first letter of whatever they are?

[Trevor]

Wow, you are just in a complaining mood today.

[Trusty]

And I've been waiting forever for the silly, I mean friendly traffic light to change.

[Trevor]

Uh huh. Well, I can tell you with 100% certainty that I'll be turning green right now.

[Trusty]

Oh wow, that was perfect timing. Like magic. Almost.

[Trevor]

Yep. Go ahead, cross the street before you get even hangrier. Enjoy your day.

[Trusty]

Uh, actually.

[Trevor]

What? What are you waiting for? You've got the light.

[Trusty]

Uh, I know, I know, but. Well, now that we've been talking, I have so many other questions. Like who invented traffic lights and how do they work? And why are the col always red, yellow and green? Why not purple, orange and brown?

[Trevor]

Purple, orange and brown. Oof, those would look terrible on me.

[Trusty]

Sorry, just a suggestion.

[Trevor]

Also, there's a perfectly good reason that red, yellow and green are the traffic light colors all over the world.

[Trusty]

Great, I'd love to hear it.

[Trevor]

Yeah. You sure you won't get too hangry?

[Trusty]

I'm sure. Besides, smarting always comes before snacking. Get ready for another whiff of science and history on who Smarted. Who's Smarted? Who's smart? Is it you? Is it me? Is it science or history? Listen up everyone. We make smarting lots of fun. But who's Smartett? Hey, smartypants, have I got exciting news for you here. I'll give you a hint. Yep, that's right. New episodes of Mysteries about True Histories are coming soon. So head on over and see what wild time traveling adventures our young problem solvers are up to this season. And if you've never heard of mysteries about true histories, AKA math, just trust me. You're going to love hearing how Max and Molly use math, history, geography, logic puzzles and fun to solve mysteries about true histor. From the creators of Whosmarted comes Mysteries about True Histories starting May 15, with new episodes every Thursday. Subscribe and download anywhere. You listen to WhoSmarted. Wow, this is a really, really busy road traffic light, Trevor.

[Trevor]

It sure is. Which is why traffic lights, like me, were invented in the first place.

[Trusty]

Aha. And when was that?

[Trevor]

It all started back in London in the 1800s. 1868, to be exact. And in front of the British Parliament buildings, to be even more exact. Back then, there was quite a lot of horse and carriage traffic crossing Westminster Bridge.

[Trusty]

Wow, look where you're going. I have the right of nay.

[Trevor]

Not to mention pedestrians being shoved off to the sides. Hello, I'm walking here. I'm walking. So something needed to be done to get things in order.

[Trusty]

Enter the traffic light exactly as it is today.

[Trevor]

No. Enter the traffic light. Very different from how it is today.

[Trusty]

Smarty pants. True or false? The first traffic light was powered by gas lamps and had to be manually controlled by a police officer. If you said true, you're right.

[Trevor]

That's right. The first traffic light was a far cry from what you see on the roads today. But it was a start. You see, before the traffic light was invented, it was the job of police officers or traffic officers to direct traffic by hand with the help of whistles to get people's attention.

[Trusty]

Right, but we still have traffic cops today.

[Trevor]

Yes, but usually only when there's a parade or event or an accident or if a traffic light breaks down. Or when there's really, really, really crazy traffic. Like outside the Lincoln Tunnel in New York City during R. But before traffic lights, it was traffic cops and only traffic cops.

[Trusty]

All right, let's go. This ain't a parking lot. Keep it moving. Okay, okay, I get it. So if the first traffic light in 1868 didn't look like the ones of today, what did it look like and how did it work?

[Trevor]

Well, for one thing, it was based on something called semaphore.

[Trusty]

Semaphore. Semaphore. What's a semaphore, smartypants? Do you know? See if you can figure it out. Is it A, a system of lights based on the number 4, B, an early type of electricity, or C, a system of signals first used on train railways?

[Trevor]

If you said C, you're right. Semaphore is a signaling system in which a mechanical arm or flag extends from a pole. That mechanical arm has some colors or patterns on it. That, along with some lights, indicates to a train operator whether it's safe to move forward, whether they should slow down and proceed with caution, or whether they should stop.

[Trusty]

Gotcha smarty pants, does that signal system sound vaguely familiar? Mm. So how did the semaphore system from the railways wind up on the first ever traffic lane?

[Trevor]

Well, just like on the railways, the first traffic light was a column with mechanical arms and gas powered lights. But the difference here is that a traffic cop would manually operate the semaphore arms. If the arm is horizontal, that would mean drivers needed to stop. If the arm was at a 45 degree angle, it would mean go.

[Trusty]

Makes sense. Of course, it might be hard to see those arms at night. This was before high powered street lights.

[Trevor]

Great point, Trusty. Which is why at night a green light would signal go and a red light would signal stop.

[Trusty]

Aha. The famous red means stop, green means go, and these signals are known all over the world. So was the reason for picking these particular colors for traffic lights because they were the ones already being used on the railways?

[Trevor]

Pretty much. If it ain't broke, don't fix it. Since red and green lights had already existed in railways semaphore for about 20 years up until that point, they just kept those same colors. But there's a couple of reasons why those colors were picked in the first place.

[Trusty]

Smarty pants, any idea why those colors were used to represent stop and go?

[Trevor]

Uh, well, for one thing, red and green are very high visibility colors, meaning they're easy to be seen at night against a dark sky. For another, they're pretty much opposites to each other.

[Trusty]

Makes sense. While purple would look cool, it would be hard to see against a purple sky at sundown.

[Trevor]

Well, there was another color used in early semaphore, but they did away with it pretty quickly.

[Trusty]

Ooh, purple. He asked hopefully.

[Trevor]

No, not purple.

[Trusty]

Okay, okay, so what color do you think that was, smarty pants? Was it A, white, B, orange, or C blue? The answer is white.

[Trevor]

Oh, white used to mean go, but because a train conductor could easily mistake it for an object in the night sky, it was changed.

[Trusty]

Smartypants, what would a distant white light look like at night? Did you say a star? That's right. Stars could easily be mistaken for white lights.

[Trevor]

Exactly. So they went with a color that couldn't be mistaken for anything else. Green.

[Trusty]

Cool. Of course, a green light could be mistaken for an alien spaceship.

[Trevor]

No, that's never been an issue.

[Trusty]

But it could be. Are you saying you don't believe in aliens? Traffic light, Trevor.

[Trevor]

I'm saying this is the traffic light episode and we're getting a little carried away. Focus, trustee, please.

[Trusty]

Okay, okay. So the first traffic light debuts in London. Outside British Parliament in 1868 using a semaphore style signal and lights powered by gas lamps being operated by a traffic cop. And then, let me guess, it becomes hugely popular and starts popping up all over London?

[Trevor]

No, it explodes. Less than a month later, it injures the police officer operating it. Ow.

[Trusty]

Oh, that took a turn.

[Trevor]

Yeah, it wasn't a bright start for traffic lights, but those semaphore lights did get used around the world for the next little while. Until 1912 when the first electric traffic light was invented.

[Trusty]

And where do you think that was, smarty pants? China, Germany or the United States? If you said the usa, you're right now, smarty pants, what profession do you think the inventor of the electric traffic light held? Was he a an electrician, b a police officer, or C an engineer? If you said B police officer, you got it.

[Trevor]

A police officer named Lester Wei improved upon the traffic light. Which makes sense since it was traffic cops who had to operate all those.

[Trusty]

Early models and got hurt when it blew up. So was this first electric traffic light like the traffic lights of today? No, of course not, smartypants. We'll find out about this first electric light and how and when the modern traffic light came to be right after this quick break and a word from our sponsors. This episode is brought to you by ixl. As someone who's constantly balancing work and life, I know how tough it can be to make sure kids get the academic support they need, whether they're homeschooled or in a traditional classroom. That's why IXL has been a lifesaver. It's an online learning program that covers math, language arts, science and social studies with interactive practice problems, games and video tutorials. Plus, it makes learning fun, just like whosmarted. For homeschool parents, iexcel is the perfect supplement to your curriculum, giving kids personalized learning at their own pace. And for parents of traditional school kids, it is a great way to reinforce what they're learning in class, whether they need extra help or are just looking for a challenge. Make an impact on your child's learning. Get IXL today. Podcast listeners get an exclusive 20% off when you sign up at ixl.com smarted Visit ixl.com smarted now to give your child the best learning support available. IXL Where Learning Clicks this podcast is brought to you by Squarespace. If you've got an idea, a brand, or a business, Squarespace can help you bring it to life online so easily and so quickly. I started with their Blueprint AI tool and it's honestly kind of magical. You answer a few simple questions about your project and it instantly builds a beautiful custom website with design layout and it even had text suggestions that went perfectly with the idea that I was creating. It's like having a designer and copywriter on your team, but it's all done by the app. Then I discovered how easy it is to offer services and get paid. You can add bookings, send invoices, accept payments, and even schedule automated emails all right from your site. No plugins, no chaos, no extra apps. And figuring all that out, just a smooth professional setup that actually works right out of the box. It saved me a ton of money and time. So if you're ready to build something real, head to squarespace.com smarted for a free trial and when you're ready to launch, use offer code smarted to save 10% off your first purchase of a website or domain. That's squarespace.com smarted modern parenting is tough.

[C]

But to raise happier and healthier kids, parents need to focus on their well being too. I'm Dr. Laurie Santos and in the new season of my podcast the Happiness Lab, I'll be looking at happier parenting. We'll tackle many of the challenges facing modern parents, from loneliness to struggles over screen time and technology. We'll explore research backed strategies moms and dads can use to achieve more joy and less stress while raising the next generation. Listen to the Happiness Lab wherever you get your podcasts.

[Trusty]

Now back to who's smarted? Okay Trevor, how was the first traffic light different than the traffic lights we're used to seeing today?

[Trevor]

Well, for starters, it didn't have a yellow light, sometimes called the amber light. It went straight from green to red.

[Trusty]

Oh, but isn't it kinded dangerous to not warn people that the light was going to change and that you should slow down before coming to a stop?

[Trevor]

Absolutely. Which is why they had this. Uh.

[Trusty]

What was that, smarty pants? Do you know? Was that an angry bee? Maybe someone getting an answer wrong?

[Trevor]

That trustee was the sound that would play a few seconds before the traffic light changed from green to red.

[Trusty]

Oh wait, they used a sound that sounds like a bad idea.

[Trevor]

Yeah, but you have to remember, even though cars existed then, they weren't very popular or fast. But don't worry, it wasn't too much longer when the first three color traffic light was invented in 1920. And with that, the amber light replaced the nice.

[Trusty]

Hey smarty pants. True or false. This new and improved traffic light was also invented by a police officer. If you said False. Sorry, I had to. The answer is true. Once again, the new traffic light was invented by a police officer.

[Trevor]

Yep, Officer William Potts, to be exact. But those new and improved traffic lights still weren't automatic.

[Trusty]

Still. Ugh. Why is history so long?

[Trevor]

Patience, trustee. You said you wouldn't get hangry on me.

[Trusty]

Sorry. Please continue.

[Trevor]

It only took another two years because in 1922, the first automatic traffic light was installed in Houston, Texas. And it changed everything.

[Trusty]

Everything?

[Trevor]

Yep. You see, once the lights became automated, there wasn't much of a need for traffic cops. In New York City alone, out of 6,000 cops on the force at the time, 5,500 got to be reassigned to other tasks. And that saved the city over $12 million.

[Trusty]

Gotcha. So how do these new automatic traffic lights work, Trevor?

[Trevor]

Well, these traffic lights utilized built in timers which controlled when the lights would change from green to amber to red. It's a simple system. It just needed electricity to make it work.

[Trusty]

And is that still how traffic lights work today?

[Trevor]

Well, things are a bit more advanced these days with traffic lights having more control. For example, if I see a lot of traffic, I can create a green wave.

[Trusty]

Ooh, smarty pants. What do you think a green wave is? Is it A, an environmentally friendly system of traffic lights, or B, a system in which several traffic lights across multiple intersections are coordinated to turn green, allowing cars to drive through without having to stop at a red light?

[Trevor]

That's a mouthful. But if you said B, you're correct. Today's traffic lights do way more than just change colors. Sensors and automation can actually help with the flow of traffic.

[Trusty]

Oh yeah? How?

[Trevor]

Well, the green wave is just one example. There's also digital sensors and induction loops. A digital sensor helps control when light should change by sensing when a vehicle is at an intersection. Similarly, induction loops are installed on the road itself and use an electromagnetic system to sense when the vehicles are on the road. Whoa. Granted, sometimes the sensors have predicted problems feeling the weight of a lighter vehicle, like a motorcycle or a bike, and wind up keeping the red light on a bit too long. But traffic lights are always improving.

[Trusty]

Gotcha. Are all traffic lights exactly the same all over the world, or are there differences? What do you think, smarty pants?

[Trevor]

Well, many are the same, but some are different. For example, if you're in Japan or the Canadian province of Quebec, your traffic lights wouldn't be vertical, but horizontal. No, but other aspects of traffic lights are standardized all over the world for safety reasons, like the order of the.

[Trusty]

Colors on vertical lights. Red is always on top Amber is always in the middle and green is always on the bottom. And on horizontal lights, it depends which side of the road that country drives on. In Canada, where they drive on the right, the horizontal lights go from left to right, red, amber, green. But in Japan, where they drive on the left side of the road, the horizontal lights go green, amber, red.

[Trevor]

Then of course, there's South Africa, where they don't call traffic lights traffic lights. They call us robots.

[Trusty]

Interesting. Well, thanks for the history lesson, Trevor. I'd love to stick around, but it's definitely time for pizza, so I'll just be going and. Oh, great, I missed the light. A big across the ocean shout out to Eliza, who is listening. Near London, England. You said you love who's smarted because it tells you the facts. Well, the fact of the matter is, you are a brilliant smarty pants. Thanks so much for listening and learning with us this episode. Traffic Lights was written by Phil Greenlight Jeremy and voiced by Mike. No purple privity. Adam, don't run the light. Davis, Emma. Flashing red Wilson. Max, stop the car. Kamasky and Jerry Colbert. Technical direction and sound design by Josh. Don't walk Han. Our associate producer is Max Kamasky. The theme song is by Brian Mixed Signals Suarez with lyrics written and performed by Adam. Tex Davis who Smarted was created and produced by Adam Tex Davis and Jerry Colbert. This has been an Atomic Entertainment production.