In my opinion, average speed is an absolutely worthless figure. Traffic can be different from day to day. Wind speed can fluctuate greatly, etc. And average speed is absolutely incomparable with someone else's unless they are riding the same route at the same time under the same conditions.

I have completed rides clipping along at 22-24 mph and only finished with average speeds of 15 mph so clearly average speed tells one little about how fast one actually goes most of the time. Moreover, most bike computers have a function that allows one to not gather any data when the wheels are not turning. This would result in a VERY different average speed than someone who did not turn this feature on. So the whole average speed thing is very inconsistant as a measure with which to compare yourself to other riders.

To answer your question, you can be moving very fast most of the time and still only finish with an average speed of 13-14 mph if you have a lot of traffic, hills or lights. Perhaps we could give you better information if you told us how fast you were moving most of the time on flats and with little wind either direction.

That being said, on my road bike I can usually clip along between 20-23 mph in the mornings and 22-25 mph at night in the abscense of real wind. My commute is flat and 10 miles each way. The mountain bike with slicks is only a little bit slower. I think, however, that I ride faster than many commuters. I have been passed once on the commute and it was very suprising (I took the opportunity to suck his wheel and make a new friend).
Troy