Admittedly, one of the only reasons I haven't gotten hit yet and why I muster up the confidence to ride fast is my hearing. I ride in a residential area, so most of the sound is the wind going through the trees, so I can always hear when a car's coming up from any direction.
But what about those of you who use your eyes instead of your ears? (Well, I guess the first question would be if anyone does that.) How do you safely listen to music and ride your bike?