Probably end up wetting myself from the shock or wrecking my car somehow!LOL
Hopefully, I could contain myself and just roll down my window and tell him how great I think he is and let him know I support him.
But if I couldn't contain myself, I would want to secretly follow him to his destination. And then, well, who knows????
For real though, I'd get out of the car and pretend I'm a local and try to get a pic and his autograph. Then tell him how great I think he is and let him know I support him.