Yelling. I can't stand it when I'm driving, especially impromptu, out-of-the-blue shouting about something.

Seatbelts go without saying. It's the law here, as well as common sense, so I don't need to mention it.

When I'm going anywhere at night, I insist on my passengers locking their doors. It only takes about half a second, so it's no hassle, but it could be what prevents some random drunk asshole from yanking a door open to deliver facepunches or grab at handbags. Sure, it'll probably never happen to us - but it *does* happen from time to time in these parts, so I make a habit of keeping car doors locked after dark.