I luckily get to commute mostly on fairly lightly traveled roads (at least in my judgement) - I do have a short stretch through Charlottesville where, for te most part, the traffic isn't too bad. My morning commute is 15 miles and, as I leave the house at 5:30am, is mostly ridden in the dark except for a few months during the summer. My ride home is on a slightly different route to avoid one busier stretch of road during the afternoon rush hour. The detour ends up making my standard commute home clock in at 17.5 miles and, for the most part, I get to run it while it's still light out (there's a short stretch in December/January where it's getting nigh on dark by the time I roll into my driveway.
During the fall/winter - basically any time I need to use my HID headlight - I commute on my Surly Steamroller fixed gear running a 46x16 gear. During the Spring/Summer I usually end up riding my LeMond Zurich and, more often than not, I will end up extending my ride home - sometimes just by a few miles, sometimes tacking on 30-40 extra miles. The only times I don't ride to/from work are when there's frozen precipitation actively sticking to the road (my wife draws the line at studded tires unfortunately) or when life necessitates the use of a car for one reason or another.