yeah, good point.... I think the difference is that when you drive a car you normally have a set lane in which you are driving, and if you leave that lane then you check your mirrors etc. You are also already cocooned within a steel cage, so it isn't going to affect your hearing that much. if you are riding a bike then I find it useful to have a better idea of what is around me as you are more manoeuvrable and more unpredictable than a car. If I can hear a taxi over my right shoulder then I know I need to make sure I stick close to the curb, and don't pull out right to avoid the puddle in front of me. Of course, you should have a look over your shoulder as well, but I just find it gives me more info to go on when I have to make a quick decision.