According to my BikeJournal.com; 2004 was 16.68 mph avg. spd. That was after 145 rides totaling 4500 miles.
Some rides I was able to come in around high 17s, most were in the high 16s low 17s. The reason for the 16.68 I think was due to a bunch with my girl friend. Those were in the 12-13s, throwing my avg. way off.