For every stupid sign somewhere, there was a stupid person that caused the need for that particular sign.
Every time someone with no common sense does something dumb, someone else has to make a sign to prevent this from happening again. This must happen more than we'd think (judging from all the signs.)
Now, I play a game with myself. Every time I see a stupid sign, I think of the stupid person that made the need for these horrible (and seemingly useless) signs.