The fastest average I have ever maintained was back in August for 20 miles @ 18mph but it was a one time affair and I had pretty good company on the ride and it may have effected the speed, but whether it was for better or worse, good company is more important
BUT
I hardly ever ride outside on long straight flats. My Average when I'm out ripping on urban streets is 15 or so. It depends on what bike I'm on, how I'm feeling, where I'm going, and how many stops I have.