In the airport you'll see signs telling you that the moving sidewalks end. Good to know; one might not notice and expect the floor to carry us onward.
In Maui we came across a sign informing us that the sidewalk was ending. Within three feet, the concrete stopped and there was a sturdy fence across the end. The sign was obviously needed to keep us from walking into the fence.
After driving on a twisty, turning road for quite a while, we kept noticing signs that said "curves ahead." No kidding, this road is famous for its curves, that's why we were on it. I even bought a tee shirt that said I survived it.
How about the signs that tell you the road has no shoulder. That's something we would not know without a sign?
I know there are lots more. I'll post again when I remember what they are. In the meantime I'll ponder why we need these signs. Are people really that dumb or is it just a result of the "sue somebody" mentality that seems to be out there?