Safety is an issue, to be sure. But I have gone back to music while running recently. In high school I created a tape with "Man in Motion" (from St. Elmos Fire) playing on continuous repeat, which lasted just long enough to run from my mother's house, in far south Anchorage, down the railroad right-of-way to the Potter way-station (now a tourist stop-over complete with gift store and museum) and back. That run follows Turnagain Arm as it extends south from Anchorage and toward the Kenai Peninsula, with (on the out-leg) Potter's Marsh on the left. The marsh is a famed bird sanctuary, with an enormous population of Canadian Geese breeding in the summer months. Arctic Terns also nest nearby, and it was not uncommon, in the wrong months (though I can't recall as I sit here what those months were) to be dive-bombed by protective parent terns. Winds blow nearly continuously from the south along the Arm, so a runner pushes into the wind on the way out and gets carried by it on the way back. I also recall the t-shirt that I favored when doing this run. It was branded by the alpine ski manufacturer K2, and read "Feel the Need for Speed. Satisfaction Guaranteed." I never clocked the distance, but this was a 60' or so run that I must have repeated 50 times during my high school years. I did use to worry that Man in Motion would obscure the sounds of an approaching train, but I emerged unscathed.
I eschewed headphones for many years after. Carrying a walkman was a pain (in the pre-iPod days), the ear-phones kept falling out of my ears as I sweated, and I told myself serious runners concentrated on their running. Recently I have had a coach who teaches the last rationale: one should use runs either for building speed, which requires concentration, or working on form, proper foot-strike, what-have-you. I followed that approach until about 6 months ago.
I'm back on the headphones. It started when I read that Craig Alexander, for the past two years the world champion in ironman triathlon, enjoys running with his iPod. I then realized that my training challenge was not working on foot-strikes, it was getting myself out the door for a run after a long day (due to fatigue), or even a lazy day (due to lethargy). The iPod is more pleasant to carry than the old, yellow, Sony Sports Walkman used to be. I've become facile with routing the head-phone cord through my hat first, helping to keep them in place. I normally run through Rock Creek Park in DC, where cars are much less of a concern than on the streets of New York or Chicago, and which I can reach with an approximately 8-minute jog from my front door. The right music helps me surmount the psychological hurdle to getting started. A little ACDC on a hill workout keeps the speed up. And I still have Man in Motion on the playlist!