Though this is not something I saw on the road, I have always loved the opening to "Fawlty Towers" where they play scrabble with the letters.

There is a bar in Baltimore, MD. called "Berthas". They have a famous slogan (there's a song about it as well) that is printed on bumper stickers. It says "Eat Berthas Mussles" (Baltimore is famous for seafood). People do much the same with the letters of the bumper stickers, and if I had any memory what so ever, I'd relate the funnier ones here.

Perhaps it's time for a road trip to Berthas!