Interesting/odd. I was just using Bicycling Magazine's calorie calculator out of curiosity, and they claim that 14-16 MPH cycling is "vigorous effort" and 16-20 MPH is "racing." I max at > 25 (up over 30 at times) and average 16.8 over my whole commute (dropped down significantly by hills and gravel road) and I don't feel like I'm even riding aggressively, I ride so that I'm just pushing enough to sweat.
( G+ comments below the cut )
( G+ comments below the cut )