I ride in urban traffic without a mirror. There are always cars passing me. If I looked in the mirror, I'd pretty much be guaranteed to see a car behind. If I looked in the mirror and saw a car head straight up my backside, I don't guess there would be much I could do about it. Mirrors are a convenience to some, and others find them a distraction. There is no one size fits all answer, and unless you have some statistics to back it up, we will just go round and round with conjecture. And as to cars and bikes, the difference in the visibility from a car, and the size of the mirrors on a car make them very different indeed. My ability to see and hear traffic from my bike without a mirror is far better than my ability to see and hear traffic from a car with multiple mirrors.