My commute is 15 miles in and 17.5 home (I take a different route to avoid one nasty stretch of road at rush hour). I maintain a detailed log of each ride so I've got good historical numbers to compare to/with. I also know the distance of my commute so I can figure the average speed using distance/time and then compare to what my cyclecomputer says.

My ride to work takes somewhere between 42min53sec (feeling really good, riding my LeMond & getting lucky with the lights as I run the last 5 miles through town) & 59min21sec (10 degrees out and riding my fixed gear commuter & hitting almost all the reds). As such, my average speed runs the gamute between 15mph & 21 mph for the commute to work.

My ride home (provided I don't extend it which, during the spring/summer/fall I generally do when I'm feeling good) takes somewhere between 56min54secs & 1hr 8min 44sec. As I generally don't end up riding straight home when I'm feeling in the zone, my average speed doesn't get up there like it does in the morning: commute home average speed hovers somewhere between 15 & 18.5mph.

I commute 5 days a week provided that there's no snow/ice on the roads (that's where my wife draws the line right now). During the winter my commute is more for base building. During the spring/summer/fall my commute ends up being more varied intensity wise depending on what my goals are, how I feel and what bike I'm riding.