This may or may not qualify me as an idiot, and the idiom may or may not fit exactly, but I suffered from a misconception while I lived in Toronto some years back, and it was my husband who had to straighten me out.
One of the first times I commuted to downtown TO, I saw a sign on the side of the road that said "No Standing." Around this sign, seated in the grass, were a number of what appeared to be homeless people. I remember thinking that it was incredibly callous of the city to put up a sign to prevent homeless people from standing on the sidewalk. I thought they were sitting on the grass around the sign in protest of some sort.
I saw this sign quite frequently, and it was some months before my husband came downtown with me, and we passed the sign. I mentioned to him how I had seen the homeless people sitting under the stupid no standing sign. "Why would they make them sit down?" I asked. "So what if they stand on the sidewalk?"
(I will mention at this point there was no such thing as a "No Standing" sign in Edmonton back then, and I don't think I've ever seen one to this day.)
Of course he had a good laugh at my expense, and told me that the sign meant no taxis, as in no taxi stand. I Googled this just now to be sure, and more accurately it looks like it applies to any vehicle, not just taxis. Basically it means no idling.
Why wouldn't they just say "No Stopping" or "No Parking" or "No Idling?"
We have plenty of those signs in Edmonchuk.