lol! So far this thread seems to comprise of 2 types of responses -
1. Of course it's possible!...but I don't do it.
2. I actually commute, and I never or almost never can do 20mph average, even though I'm in great shape, bike regularly, and really push it some days.

At least for replies that are talking about "average" speed is "the entire average speed from the time you leave to the time you arrive at work, including time spent sitting stopped at the stoplights, slowing down for obstacles, etc etc".