I sometimes use a radio or MP3 player while commuting. My ears stick out a fair amount, so if I do not wear headphones, I have very loud air rushing noise in my ears. I wear Sony "street style" behind-the-neck open-air phones; with them on and the radio on just loud enough to hear, I can hear better than without the headphones. I have tested by looking how far away cars are when I first hear them, and I can hear around 1/4 mile with headphones, just a touch less without. That's for passenger cars. Pickups obviously sooner; usually I hear them before I can see them (around corners/over hills).

Playing music loud enough to drown out ambient wouldn't be too smart IMHO, but I also think that of radios and CD players in cars, but people do that too.