When we think of dangerous driving music, you might imagine rock-and-roll (rather than, say, classical opera), but a study out of Memorial University in Newfoundland suggests that the volume, rather than the kind of music, is more important for safety.
In the study, any kind of music became distracting right when the volume was turned up. This makes sense; the more things demanding your attention, the longer it takes your brain to process to interpret them all.
Sometimes music can help keep us alert and sometimes it can help us relax—but beware getting so relaxed you begin to lose your sharpness.
When driving, stay focused on the road, not on the tunes.