I was thinking about that; like, "Ever hit who from behind?"

As far as music players are concerned, I don't think that any hazard can be heard well enough and far enough in advance to be avoided by audio cues alone. If I'm going to cross three lanes of traffic, I can't hear the sound of the slowpoke drivers who hang back over the noise of the pack of closely-following drivers that just took off from a light. That "pack" is easy to avoid, because they're all in one clump -- but it's the other cars off by themselves that I need to carefully look for.

So, for me, and ironically enough, busy traffic is where it's fairly useless to try listening for hazards. Seeing things with my own eyes and with a mirror, plus riding predictably, is what keeps me healthy.