---
title: 'Why Stop Signs Are Red: The Science of Retroreflection'
source: 'https://youtube.com/watch?v=4iY8JqHN-kI'
video_id: '4iY8JqHN-kI'
date: 2026-09-09
duration_sec: 576
channel: 'AsapSCIENCE'
---

# Why Stop Signs Are Red: The Science of Retroreflection

> Source: [Why Stop Signs Are Red: The Science of Retroreflection](https://youtube.com/watch?v=4iY8JqHN-kI)

## Summary

This video explains why stop signs were originally yellow and how retroreflection technology made red signs possible and visible at night. It covers the science of color perception, the invention of retroreflectors, and how 3M solved the rain visibility problem.

### Key Points

- **Yellow Stop Signs** [00:00] — Stop signs were yellow in the 1920s because red pigment faded and red is hard to see at night.
- **Why Red Is Hard to See at Night** [00:26] — Rods in the eye don't detect red wavelengths, making red the hardest color to see in low light.
- **Peak Eye Sensitivity** [01:00] — Our eyes are most sensitive to yellow-green, peaking at about 550 nanometers.
- **Retroreflection Invention** [01:12] — 3M invented retroreflection, using glass beads and cube retroreflectors to bounce light back to the source.
- **Cube Retroreflector** [02:29] — A cube retroreflector with three mirrors reflects light back to the source regardless of angle.
- **Retroreflector Density** [03:29] — 3M's retroreflective sheeting contains about 6,000 retroreflectors per square centimeter.
- **Sign Manufacturing** [04:06] — Signs are made by cutting retroreflective sheeting, applying letters with a plotter, and using pre-space film to keep pieces aligned.
- **Digital Printing** [05:19] — Digital printing uses water-based HP latex ink on retroreflective sheeting.
- **Rain Problem** [06:54] — Rain lowers the refractive index of retroreflectors, causing light to scatter away from the source.
- **Night Driving Statistics** [07:18] — Only 25% of driving occurs at night, but 55% of fatal accidents happen in darkness.
- **Dual Refractive Index Solution** [07:32] — 3M uses elements with refractive indices of 1.9 (dry) and 2.4 (wet) to maintain visibility in rain.

### Conclusion

Retroreflection is a brilliant engineering solution that makes road signs visible at night and in rain, dramatically improving road safety.

## Transcript

Did you know that stop signs used to be yellow? Back in the 1920s, even though red had already become known for stop in traffic lights, the convention in signs wasn't yet established, and there were two major problems with red.
One, there wasn't yet a way to make red pigment that wouldn't fade, and two, red is way harder to see at night. You see, your eye has three types of cones that detect color, red, green, and blue cones. But at night, there isn't enough light to activate these cones in your retina,
And so your eyes rely on rods, which are much better at perceiving low light. And if you look at the wavelength sensitivity of rods, you'll notice they actually don't even pick up red wavelengths. When you see red at night, it can only be picked up by cones, and as a result, red is the hardest color to see in low light.
So while a traffic light shining red is one thing, a lowly lit red sun is completely different. On top of this, even in daylight, our eyes are actually most sensitive to a yellow-greenish color. This is because it's rightly overlapped between our red and green cone sensitivity,
peaking at about 550 nanometers, making it a perfect option for visibility, but not exactly one for consistency with red meaning soft. So when did the switch to red signs happen, and how did we overcome this hurdle?
It's actually one of the most ingenious and coolest inventions to ever come about, retro reflection. I'm here at 3M's Innovation Theater, the home of the retroreflector, where it was invented, created, and first implemented.
And we're going to talk about what they are and how they work. I have a little laser light here, but to not blind you, we understand if a light comes into the mirror, it's going to reflect back into this lens or your eye. But if you point the laser at an angle to a mirror, it exits at the equal angle the other way.
As you can see over there, I have that laser pointer on the shelf. And that's not very useful if you're trying to get light reflecting back to someone on a sign. So we need a better solution. And 3M came up with that many decades ago. They first used these glass beads.
Now, when light enters glass, it's a different refractive index, so it actually bends the light. If we put this down here, it's a white surface that's reflected behind it. And I shine this laser through it.
You can see that laser comes back to me now. Instead of bouncing out the other direction, it's actually shooting back at my stomach. A little faint but still there But 3M took that principle even further with a full cube retroreflector If we take this and just talk about these first two mirrors here if I shine a laser in one side
of that mirror, it will bounce out at the equal and opposite angle, and then do it again, and you can see on me, this laser looks basically just as strong as it did going in as it does on me. So that's really useful that it's reflecting back to the source where it's coming from. And when you
think about signage, you want the light to hit it even if it's at a weird angle and come back to you so you can see the thing. By adding a third mirror down here, we can now basically point anywhere on here, and we're going to have reflection that ends up back towards me. You can see it here as well, even though I'm pointing on the bottom.
And that is the basic 101 of how a retroreflector works. Now the question is, how do we go from the principle of retroreflection to getting it on a huge variety of surfaces, like not only signs, but multiple plastics, the lines on the road, and even clothing?
Walk me through what the heck you are moving around here. These are all reflective shootings. What we have here are different type of reflective shootings. So these are all basically full of retro reflectors, right?
Correct. This is full of retro reflectors. Basically, each of these little squares has 6,000 retro reflectors per square centimeter. Yes, you give their a home work. Pretty good, then.
You got it. This though is like a metal backing of which these retroreflective sheets are put onto. Sorry, correct. The most important sign of the rule. Yes. Not to buy the sheets or materials.
I wasn't about to leave 3M without a song of my own, so I made sure to ask specifically how the songs get put together. We're going to start with the retroreflective sheeting. So we'll cut it to size. And our retroreflective sheeting is basically a big sticker. So there's a liner and adhesive on the back.
So once you apply it on here, you get kind of this lip, this overhang. So we have to trim that off, and then separately, we'll take the electrocog overlay film, or ECOG as some people call it, and we'll put it through a push-and-send plotter.
We wrote A-S-S-I-N-E. Yes, I hope I get to keep that. You just kind of grab the edge and just push it, and then you can just lift the letter off. Whoa. You can't pull it off. Okay, there we go. All right.
Some letters, like the A, will actually have a little floating piece right in the middle. And so applying this pre-space allows you to keep everything in place and remove the liner. And especially if you end up with something like this.
So you remove the liner the green is stuck to the white retroflexive sheeting it on aluminum and the last thing to do is to remove this pre Ooh I would love to Are we ready
Oops. Wow. Look at that. Taste that science. Almost every street name time is made like this.
Before digital printing. That's crazy. And another way to manufacture science is using digital printing. So digital printing has been around for about 10-15 years. I read it three times. So this is printing on retroreflective sheeting.
Exactly. This Pacific printer uses HP latex ink, so it's water-based. So it's similar to like your hydrant film, which is flamed-out ink. All right, can we see a little something, something? Absolutely.
It's morning, I'm sorry, I'm sorry. That's so cool. Boom. Okay, we're outside in the dark, and we're going to do a little experiment with the sign now to see it in action. So if I shine this flashlight on the ground, my immediate surroundings are pretty visible,
but become increasingly less visible as we go further out, and that's because light scatters in all directions, and less photons are actually making it back to the camera lens. But if I shine this light now over towards Greg holding our fancy retroreflecting sign,
and put the light right beside the camera lens, you'll see just how much brighter the sign is than almost anything else the light is touching, even though it's all the way over there. Now, of course, if I move the light away from the lens, the sign becomes less bright.
Even here, I'm about six inches, go a little further by the foot, you can see how much darker the sign is, and that's because the camera lens, or you, are no longer the source, and instead, the majority of the light is bouncing back over here.
Over here, while I'm holding the light, I see the sign just as bright, but you don't, and that's because the light is always reflecting back to the source. You've likely seen this while driving at night when there are minimal overhead lights. Signs super far in the distance are extremely bright, while things closer are completely dark.
Even the metal post seems invisible here. Now what about when it's raining outside? Sure, in the broad daylight you can still see the lines on the road, but I think we're all familiar with that scary problem of lines disappearing at night.
The problem here is that the water actually changes the refraction of light. So, where the retroreflectors act ideally under dry conditions, As soon as water is on top of them their refractive index is lowered by a ratio of 1 and so the light leaves at a different angle sending less back to the source And this is incredibly important because while only 25 of driving occurs at night 55 of
deaths from car accidents happen when it's dark, with rain being disproportionately dangerous at low light conditions compared to the day. To combat this rain visibility problem, 3M actually developed these elements, which are retroreflectors that have different refractive indices.
So on this side we have elements that have a refractive index of 1.9 and they work well in dry conditions. And on this side we have elements that have a 2.4 refractive index and work better in bright conditions.
And on top they're both in air and on the bottom they're both submerged in water. Now I'm going to show you what happens when we're in the dark and shine a light on both of these. You can see that the one on the left at the top is optimized for dry conditions because
it's flashing back more light. And then the one at the bottom on the right is optimized for water and is sending more lightback press even though it's the Merchant line. Now of course the best solution is to include a mix of these elements in the product, whether it's the layer put on top of painted lines
or directly into taped lines so that people can see them in either weather condition. If it's dry the 1.9 will send the light back while the 2.4 won't and when it's wet the opposite occurs. So behind me we have a variety of painted lines, some have the regular glass beads on them
and other ones have the 3M elements on them with the 1.9 refractive index or the 2.4 refractive index or a combination of those, and we're going to see what happens in the dark when they get rained on. Here's the setup in the dark before the rain machine has been turned on, and you can see all
the lines are equally visible, but as the rain begins to run, some of the lines slowly start to become obscured and harder and harder to see. It's only the few stripes on this right hand side that remain visible, as they have varying amounts of the water reflective element. And here's the difference
in the real world. On the left is a wet line without water reflective elements, and on the right you can see it with these elements, and as a result you can actually see that line even in the rain. So there you have it, road signs are officially way cooler than
you probably thought, or at least are a lot cooler than I ever realized they were before making this video, and hopefully I've convinced you of that too. I want to send a huge thank you to 3M for having me at their facility, and to JC and Gus for showing me around and
teaching me and especially making me multiple signs that I could bring home back here to the ASAP Science Headquarters. I will cherish it forever. Thank you so much for watching, make sure you like the video, subscribe if you haven't already, and we'll see you next time for
some more science. Peace!