Unnecessary Warning Signs Make Us Roll Our Eyes
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.