In California, it's illegal to do this. We call it a speed trap, even though that already means something — cop hiding in some shitty spot where the speed limit suddenly and unexpectedly drops or whatever. I got busted with one of these in Jackson City, TX, a trivial little carbuncle on the asshole of a slightly shortcut route to Austin if you're heading East on the I-10, with a stop sign completely enveloped in a fucking tree that probably produces 50% of that shithole's revenue.
*ahem*
Anyhoo this CO scheme wouldn't be legal here in CA. And we'd also make a city cut a tree back if they wanted to keep writing speeding tickets based on a sign inside of the fucker.