Some signs make sense, especially where the observer might be enticed to an area or activity that might prove harmful. Signs like "no skateboarding" posted on a parking lot on a slope (the businesses owners could be liable if someone gets hurt), "warning: guard dog" would be useful to a would-be thief or "speed limit enforced by aircraft" might be all someone needs to avoid the temptation to speed.

Thank God for the "no swimming" sign in this photo because I can just imagine the hot, sweaty, parched folks walking by this inviting-looking paradise. All I need is a lounge chair and a mojito to kick-start the good times! In reality I'm not sure a sign telling you not to swim in an abandoned pool is necessary. I think a sign that says "yep" (in the way one would react when seeing a sumo wrestler in a feather boa preaching the word of God in a mall food court) is all that is needed here.