Having ear buds in your ear does not necessarily mean that you can't hear. Nobody says you have to crank the volume up so loud that it drowns out environmental sounds. If you turn the volume up to where the apparent volume is the same as if you had a speaker on the bars, it should be no different one way or the other.

Some people use MP3 players with built in speakers, such as the Zen Stone.

I listen to NPR and audiobooks on my commute, and with one bud in my right ear, by my actual testing, my ability to hear approaching cars is not at all diminished. I can still hear them before I can see them (on long sparsely traveled country roads) and can hear them at least 15 seconds before they get to me (in town).