When my commute was a lot longer -- 38 mi r/t -- I would usually average 13mph at the beginning of the season, end up around 16mph in the Fall. Along decent flats, I'm usually 17-19mph; if I have the wind at my back, sometimes in the 21-23 mph.

When doing estimates for how long it will take me to get to the bus or work, I'll usually compute distance/time at 12 mph. Just to give myself a little margin. Fastest I've ever gone is low-40s down this one hill... Whenever I get to hyper-legals speeds, I always hope that's the day a cop is sitting somewhere and gives me a speeding ticket. And also, past the speed limit, I start singing Judas Priest's "Breaking the Law" in my head.