Well, it looks or sounds more like something I've been noticing with lotsa sign these days, including some of our own...... a type of mold that is not washable. I believe there is a kind of fungal transitioning due to possibly incompatible inks with laminate, not enough drying time before laminating, to morning dew not evaporating or just which way the sign faces in traffic.
The traffic one could easily be the exhaust as vehicles go by traveling up in the air and clinging and finally taking hold from air flow of the direction.
I'm noticing around us, that trees along the sides of certain roads and highways the leaves are dying off long before Autumn gets here.... and it seems to be the foliage closest to the roads. Whether or not stacks from freighters or just exhaust fumes are causing this is not known, but I never saw this stuff years ago. I originally thought is was from snow plows and salts, but they aren't plowing near as much anymore and putting down a brine which doesn't work the same way. Also, you don't see the same effects on inner city streets, just heavily traveled roads.
Just a thought.............
