I have a 1.8 mile commute in the morning that I do at 5:30 so traffic isn't an issue. In the afternoon I use a combination of an old railway spur path, a warehouse parking lot and then a quiet street. It's only a mile that way, and I avoid the busy street that my morning route becomes.

I ride in work clothes (cargo pants or shorts, t shirt, work boots and a jacket if necessary) and I have a rain suit if I need it. My commute bike is a cheap 18 speed MTB that weighs about 36 lbs with fenders, lights, lock and pump. I don't get nearly enough riding that way, so when I get home if I want to ride further I get my 2009 GT Outpost.