Originally Posted by SPlKE

Here's the dealie:

If you can't know what's behind you at least every 5 seconds (more frequently in traffic) -- by turning your head WHILE holding your line -- you may not ride without mirrors, and you may never, ever ride with headphones, not even one in one ear.

If you can maintain visual 360-degree situational awareness at least every 5 seconds (more frequently in traffic) WHILE holding your line, you don't need mirrors, you're cleared to ride with sound-isolating headphones in both ears, cranked up to 11.

Also note -- if you're the fastest thing on two wheels -- you're always passing other riders, but never being passed -- you can bend the rules above ever so slightly, as long as you can hold your line and not be a menace to any other biker, walker, driver, etc, you come in contact with.