From almost the start of bike commuting 4 years ago, my average has been about 15mph going to work and a bit slower coming home. I originally rode 16.5 miles round trip, and this year have extended it to(usually) get in 20 miles round trip. Anyway, it doesn't seem to matter if I ride the converted mtb commuter with panniers or the road bike, the average is between 14 and 16 mph going in and after the 12 hour shift, it's always slower going home(12-14), but no surprise there!