Yeah, panniers seem to make a difference,too.

Maybe when drivers see you loaded down with a pannier they trust that you will be holding your line and not hopping curbs or splitting traffic. You're haulin' that luggage on your rack, so you won't be likely to do any circus tricks!

When I'm driving and see someone who is clearly a commuter, I feel safer about their potential riding behaviour. Like I can trust them to behave like a vehicle, cuz they look more serious?