Geez--usually speed posters are one's I can't relate to.
My average is a little slower--around 12--but I'm sure that depends on your route and stops. I typically go about 16 or so on the flats. Doesn't seem that fast but I also pass most riders. Still, I get passed once in awhile, but almost always it's by a roadie all dolled up.
No doubt I'd get passed more often if there were more riders!