You ever burst out laughing while driving only to find someone is watching you?

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Yesterday I was driving down the street and at a stop sign I saw a sign that reminded me a friend and a funny situation they had. I burst out laughing and then noticed the guy in the car next to me was staring at me. It actually made me laugh even more. I am sure he thought I belong in an institution. I was really glad I didn't leave in that town so was sure he would never see me again. Have you ever done that and if so what did you do when you saw the person watching you?