It's probably the element of surprise that's the most effective here. And yes, I'm stealing and using this, big time!!!
Check out this genius way this driver attempts to merge in a traffic jam. He surprises the driver behind him with a politely-written FLASH CARD asking if he can move into their lane. When space is made, he flips to the next card that says "Thanks." (I guess you'd call that a courtesy card... Hahaha!! I kill myself.)