Clean Idle just means it's better compared to what it used to be, not that it's actually clean. An APU does tend to be dirtier on a volume basis, but it uses so much less fuel that at the end of the day it's spewing out less pollution.
Many "Dictatorial" jurisdictions with 5 minute or such idling laws will just tell you "Sticker Schmicker, you can idle for 5 minutes. It is really just a compliance thing that manufacturers in multiple industries have to do to keep the EPA and Uncle Sam off of their ###. Part of that " compliance", is greasing the palms of the proper politicians and lobbyists. Washington DC is rotten to the core. All of them. Dems and Republicans alike.
Not necessarily. Depends on the jurisdiction you are in. They won't give a crap about the sticker. I'm not saying it is right. Just saying that there can be some big fines and consequences for idling it the wrong places, sticker or not.
I have never seen a "No Idle" sign at a truck stop, but I don't drive all over the country, mainly from Indy down.
I just leave everything on 24/7. One time recently I went home for 2 days because no load. Just left the truck running. Lol. Even while fueling. New York State, same thing. Never shut the engine off.
You don't get up to NJ or PA much then. Signs all over the place saying no idling more than 5 mins. Trucks still idle though.
So I get to listen to some a-hole mowing his lawn 10 feet from my truck all night long. All the while pumping diesel smoke into my truck. Thank you @Dave_in_AZ and @otterinthewater My children will thank you for helping the environment while killing their father. It's a win-win kinda thing. But, hey, as long as you're comfortable, who can complain.