I’ve heard they place the signs along trails of droppings.
Most of the deer I’ve encountered are definitely not at the signs, though.
One particular route I take (less at night because of this) the deer stand in the tall grass and watch the traffic, about 6 to 10 feet off the road. You’ll see a family, 3 of them (one small so it looks like a family) just off the road watching the traffic. Ran into this this going through Oklahoma a few years ago too.
I saw a deer at the side of a busy four lane (two in each direction) road in CNY standing at the side of the road.
He kept looking both ways, waiting for it to be clear to cross.
My daughter and I could not believe it.