NO HEADPHONES WHEN DRIVING. ONE EARBUD ON PHONE OKAY.This is important. Just because we've said that hands-free driving is okay (Is it as safe as undistracted driving? probably not, but a conversation can be a good way to stay awake and alert, I've found.) doesn't mean that you can rock out to your tunes on your headphones. That's a primary offense and can get you a citation. And, jesus, people, haven't you ever heard of a stereo? Or are your earbuds that good?
But here's my message to whoever will listen (we'll file these under Ari's Friendly-if-Somewhat-Passive-Aggressive Helpful Hints to Cyclists, along with "For fuck's sake, wear a helmet!" and "You're riding the wrong way down JFK Street in Harvard Square in rush hour? Seriously?"): Wearing headphones on a bike is ridiculously dumb, and it's against the law, too. When you're in a car, what you're listening for are sirens, car horns and, well that's about it, everything else gets drowned out by car noise.
When you're on a bike, you have a lot more potential to use audio cues. I've found that I actually can get more information as to the presence of vehicles by listening than a quick glance over my shoulder. I've heard other cyclists overtaking me (this happens more often on Hubway bikes than it does on real wheels), I've heard cars starting, I've heard any number of cues which affected how I cycled. And if I'd had earphones in, I would have heard none of it.
Yet so often I see people riding with earphones in. I sort-of-kind-of understand this in two situations. First, if you are a professional cyclists and your coach is telling you things in a race. Still, you don't have two. Second, and this is the sort-of-kind-of, if you're on a segregated bicycle facility (bike path) and there aren't many other users and you're going faster than them. But if that's the case, just deal with listening to nature for a few minutes.
And a quick note: if you're riding a fixed gear bike with no brakes, at night, not wearing a helmet, wearing earbuds without anything reflective and with no lights, only about half the things (brakes, lights, reflective) are illegal. But THEY'RE ALL STUPID. Colin Reuter made some similar points a few years back, so go read that.