This brings to mind one that I wish I would've taken a picture of.  A couple of years ago, my wife and I were driving through the Victoria Mall parking lot and noticed a STOP sign that someone had spray painted the words "Hammer Time" underneath the word Stop. 

Hilarious.

Here's Fifteen signs that were improved with good, old fashioned graffiti.  Click the link.

http://FunnyOrDie.com/m/67aj