I play in the car, but mostly noodle around with 1st position stuff and no hand effects. I use lip block one handed as opposed to pucker which I use most of the time. I guess what I would say is that I wouldn't "practice" in the car. Practice to me is too thought intensive. I don't think that noodling is any more of a distraction than talking to a person in the passenger seat. I do, however, try to remember anything good that I come up with. If I would do one thing different it would be to have some way to record those cool little things that happen to be flushed out later when I can "practice".