Just checked my computer, and my average for my round-trip commute yesterday was 10.9 mph. I can get up to 29 mph going down one of the hills that goes for 3 or 4 blocks before a stop sign, and drop to about 5 mph going up the same hill.

On the flat bike path with few pedestrians/kids/dogs/riders I can get a faster average.

Really, 13.5 mph sounds pretty good to me.