I once saw a guy drop an empty crisp packet out of his car window while he was waiting at the traffic lights. I handed it back to him and said, "Here, mate, you dropped this!"
To be honest I'm not sure what I would have done if he got out of the car but fortunately the lights changed and he had to drive away. (And yes, he probably threw the packet out of the window again but I felt I'd made my point.)