My average speed my first week of this commute was 14.7mph. Now, 1200mi later (nearly 18mi every work day) my average speed this past week has been 17.7mph. Training does help!
(My computer is calibrated first by looking up tire size to enter the circumference in mm, then fined tuned it by putting paint on tire and rolling it out with me on bike and measured distance between dots on pavement)
Al