I hate to rain on everyone's parade of self righteousness, but I fail to see how listening to headphones is different from just cranking your car's stereo. Hearing is almost completely inconsequential to driving. When was the last time you heard someone signal anything useful with their horn? Emergency vehicles have flashing lights which are much more noticeable than sirens. Many modern luxury cars also isolate you to a remarkable degree and AFAIK safety nazis don't have their panties in a bunch about them. Finally, deaf people drive all the time without incident.
If you get into accident while wearing headphones it will be because you were paying too much attention to the music instead of the road, not because of the way you were listening to the music. If you're going to condemn using headphones while driving you'll need to condemn car stereos too. And traveling with passengers, and cup holders, and drive-thrus, and roadside billboards, and lots of other things that are more distracting. That's at least a logically consistent stance, if not a popular one. I'm willing to take this back I someone has a few studies to back them up, or at least propose some mechanism of action I didn't already eviscerate.
That's why I only use my car stereo when I have the windows open. Even music at a moderate volume is enough to block out the sound of an approaching emergency vehicle.