The ambulance crews noticed that a lot of people who wouldn't move over also had their car stereos at a very high volume. So high, in fact, that they couldn't hear the sirens blaring behind them. That's a very definite public safety issue.
It could also be argued that it's a distraction for pedestrians because a loud obnoxious car stereo detracts from their situational awareness. You're not supposed to walk city streets or drive with headphones, so why is it okay to play your stereo so loud that you can't hear anything going on outside the car and cause other people to not hear things around them?
It's nap time for me right now, so I don't feel like getting out of bed to look it up exact dates, but I left Northern VA (Vienna) and worked in D.C. something around 16 years ago. I had lived there somewhere around 8 years.
I almost wish I had stayed, as my job took from there to Filthydephia, then the fiance (ex) took me to Boston.
Trust me, it only gets worse the further North you go.