Why modern ROAD SIGNS don't need LIGHTS anymore 💡

Learn how retroreflectivity makes road signs "glow" at night without lights, all thanks to your car's headlights. This not-so-simple technology is why stop signs are red and not yellow.

-------------------------
Additional reading:
-------------------------
Basics of Retroreflection Course

History of the MUTCD

-------------------------
Time sections:
-------------------------
Stop Signs Were Yellow (0:00)
Light Scatters (1:37)
Light Reflects (3:14)
Light RETROreflects (3:50)
Beaded Sheeting (4:31)
Better Beaded Sheeting (6:16)
Cube Corners (7:21)
Shhh!... It's a Secret! (8:39)
It Works Too Well (10:00)
Goodbye to "Lit-Up" Signs (11:23)

[philliptaylor]

i just want the painted lines on the road to be visible at night and in the rain

[gali]

There's a great example of highway sign reflectivity in the Florida Keys. There are speed limit signs that say "speed limit 45" and "night 35" just below it. The 45 sign is only half reflective so that only "speed limit" is reflective but the 45 is not. The "night 35" sign is fully reflective. At night, all that can be seen is "speed limit night 35". Even when I'm right up at the sign, I can't see the 45 part at night.

[lawnmowerdude]

I remember having a candle. A literal candle that lit up a speed sigh from 60 feet away. It was amazing how much it lit up.

[MySparkle]

If you want to get into the weeds, technology connections spent almost an hour taking about this tech.

[JailbreakTips]

physics lesson AND a road infrastructure video combined in one? What a great way to start my Friday!

[gali]

60 years ago when I was a kid growing up in California, I remember the cat's eyes in the signs. They didn't work very well, but I could see them with very little problem. My 40-year old mother, on the other hand, could hardly see them at all. So since I was the oldest in the family, I was given the front seat and tasked with reading the road signs to her on long trips (such as San Jose to Los Angeles) where she wasn't familiar with the road. After a few trips, I graduated to Navigator and, armed with a road map and enough sugared road snacks to keep a rock awake, told her where to turn to get us there (just call me Garmin :). Today at 68, I can still see the road signs very well thanks to the road sign technology, and have no problems with my road trips between Phoenix and Boston.

[coastofkonkan]

Rob shed light on a topic that most drivers were in the dark.

[eaglescout]

Fun fact: the Apollo crews left retro-reflectors on the moon at the landing sites. So, the ultimate moon landing hoax conspiracy theory debunker is the ability to shoot a laser at the moon and record how much of that laser comes back, which is none due to scattering. But, if you aim it right at the coordinates of one of the retro-reflectors, you will measure a return beam of light, possible only if man had actually gone up there and put those retro-reflectors in place.

[murrethmedia]

It's cool because it's basically the opposite of Stealth technology. In stealth they try to have radar bounce away from the source, whereas with road signs the goal is to bounce the light back towards the source.

[IamAllanC]

Rob is the retro-reflective spot in my day.

[dippitydoree]

Yiour videos are fantastic! Such an engaging way to present a seemingly mundane topic. Thanks for sharing man. Stay great!

[heronimousbrapson]

On country roads in Alberta, Canada, stop signs are frequently accompanied by a flashing, battery-powered red beacon, which is recharged in the daylight hours from a small solar panel.

[LFTRnow]

BTW, there's a retroreflector on the moon which we put there decades ago. You can't see it with most standard telescopes, but this is how distance to the moon is now measured very accurately. Bounce laser pulses from a high-power laser, then check for relections and time between them.

[markpfeifer]

I learned something today. Thanks, Rob.

[BradHouser]

I drove on I-80 from PA to Wyoming several times in the early 70's and I recall very few retro-reflective signs along the way. They stood out and were colorful, which helped break the monotony of driving across Nebraska at night.

[andrewfidel]

The other important point of ditching the bulbs other than resource and money savings is it reduces the amount of light pollution our cities create.

[jamesrea]

It’s amazed me for a long time that I could see the reflection of my turn signal in a road sign hundreds of feet away. I had a little bit of understanding of how that was done but now thanks to you, I really understand it completely. This is your best video yet!

[socool]

“One of the losers in terms of technology are these old-school backlit street signs”. I invite you to Michigan, my friend. We simply MUST have backlit “only”, “one-way”, “no left turn”, etc signs in traffic signal setups.

[shiniesglitters]

The material we use to make signs in our shop is the 3M DG Cubed prismatic retro-reflective sheeting. It is on a whole level of its own compared to all the avery sheeting, and even the other 3M sheeting, and the SF price isnt super bad at about $2 per SF currently when bought in 48"x50YD rolls. A 10 year old sign with the 3M DG3 sheeting is brighter than brand new signs made with Avery sheeting. The opportunity cost though is the 3M sheeting is SUPER hard to cut compared to the Avery, we have to use a special hot tip machine that uses a carbide tip at just under 1, 000F and about 450grams of force to cut the 3M sheeting and it can only cut at about 5 mm/s

[jonahansen]

What a great story about innovation in an engineering approach setting (by engineering approach I mean a system where problems are iteratively tracked downed, diagnosed, and solved towards an end goal).