Every so often, I go out to the bike trail, ride just out of town (where it's less busy), and try to see how fast I can go for a given distance. Lately, the distance is 15 miles.
In May of 2007, I did it for 16 miles, and finished a few seconds over an hour, for a speed of just under 16 mph.
In August of 2007, I managed 17 mph for ten miles.
On my bike journal, it says I did 17mph for 18 miles on the trail in October of 2008. (And that I did 16.7 mph for 16 miles on a rolling hill road, which must have been a heck of a ride for me.)
Today I did 17.4 mph for 15 miles on the trail.
To put this in perspective, if you watched Alberto Contador attack up that really steep mountain to gain the yellow jersey in the Tour that day, they were riding up the hill at about 22 mph when he started his attack, so he went faster than that(according to what Versus was showing on the screen). (And that's up a steep mountain after riding probably 80 miles that day.)
Still, for me, this was a good ride.