I actually track stuff like that on bikejournal.com and for the month of June, my average speed was right at 17. More than 80% of my miles for the month were commuting miles, so I have to think that my average commuter speed would be right at 17. I commute about 7.5 miles each way on a Bacchetta Giro recumbent. It may be a bit faster in July as I have just changed the tires to 26"x1.125" Gatorskins running at 105psi.

Tractor Tom in Okeechobee, FL