What Causes Signs to Fade?

[Amaury]

And I know the most obvious answer would be the sun, but the reason I ask this is because I watched a video on how signs are made, and it mentioned that an extra protective layer is added to protects signs from the sun. Specifically, this video:

https://www.youtube.com/watch?v=QWyB1_TKyg0

Now, this video is from Topeka, Kansas, but I imagine this is a nationwide thing that applies to state DOTs, county DOTs, and city/town DOTs.

So, my question is, if a protective layer is applied to the signs to protect them from the sun, what then causes some signs to still fade?

Some examples from my state:
https://maps.app.goo.gl/xowLGGWPvLXgWVEz5
https://maps.app.goo.gl/BhN3CD5Ygaj1efko6
https://maps.app.goo.gl/ZUXWgZHDUzD3TfcGA

The Interstate 90 shields, though I don't know if that's printed with the sign of it's stickered on separately, which maybe explains it.

https://maps.app.goo.gl/maT2o4zcnSiEM5459 The Interstate 82 shields, like above.

https://maps.app.goo.gl/mgdGQ6FoTen58Sba9 Like many of the interstate reassurance shields, and why is it only the red part that says interstate? For comparison to a non-faded one: https://maps.app.goo.gl/Vq357YPfmjBXNHPNA

The I-90 one, installed in 2010, looks to have replaced a previous I-90 shield that was fading, looking at earlier captures. The I-82 one looks to have been installed in 2007, and the 2018 capture is the first capture with it being faded, though not as much as now. These are just a couple examples, of course, as there are many more interstate shields like this, with the red part faded.

This one isn't faded, but it's sun damaged: https://maps.app.goo.gl/sSYSM2BoB72A7pce6 Better seen here in an earlier capture before the posts were replaced: https://maps.app.goo.gl/mGDJbhtnm3VoYj8p7

This one is both faded and sun damaged and can't even be made out at night: https://maps.app.goo.gl/AfgYJWsjc2C86rs47

It doesn't look like it in the Google Maps capture, but it's easy to make out during the day in-person, so here's a capture from last year, which is what it actually looks like during the day: https://maps.app.goo.gl/xrX934VhsUfKCwxo7

[Scott5114]

I believe fading is specifically caused by exposure to UV radiation. The sun of course puts off quite a bit of this. Adding a UV-protective layer helps, but probably can't stop 100% of UV light and still be transparent. Such a layer is also not necessarily a part of every sign made (it surely costs money and we know how some DOTs are pressured to cut costs).

Beyond that, there are two different kinds of "color" on a road sign. The first is the color that is part of the sheeting. So this would be the yellow on your warning diamonds, the green on your freeway guide signs, and so on. These colors are generally quite a bit more durable because they are protected by several layers of material, most of which will degrade before the color does.

Then there are the colored inks that are applied at fabrication time. These sit on top of the surface of the sheeting. They are normally printed on white sheeting, and are translucent to allow the light to be reflected through them against the white sheeting. These tend to fade both because of their translucence and their direct exposure to UV light. (Black inks, which are totally opaque, tend not to fade.)

[Road Hog]

How does UV radiation affect north-facing signs? They are shielded from direct sunlight most of the time except for summer when the sun curves around the horizon to set. I just assumed normal weathering happens over 10-12 years in the elements.

[kalvado]

How does UV radiation affect north-facing signs? They are shielded from direct sunlight most of the time except for summer when the sun curves around the horizon to set. I just assumed normal weathering happens over 10-12 years in the elements.

There is certainly some scattered UV (why the sky is blue?); there is ozone with somewhat similar effects.
You can describe these as "normal weathering", because that's the generic description. But underlying mechanisms would include high energy photons and free radicals as important reasons.