As a teenager a friend and I walked back home from school. We saw the police installing a radar trap and decided to turn around, take our drawing blocks (we had art that day) and wrote "Attention! Speed trap!"
Result: Some of the drivers stopped, gave us some money or just said thank you! After a while (less people went into the polices' trap) the police send a patrol ... and found my friend and me with our warning signs ... lot of trouble, but the hell of fun on that afternoon for both of us (besides the philippic of the police and our parents ^^)