Besides conditions, I really think the emphasis needs to be on ride length too. I'm sure I could hold an 18mph average over a time trial course - something like 10 miles. No way could I hold that on a century. When people post their averages without their total mileage, the number is useless.

Hey - I did an 87 mile ride last week with an 18 mph average. Unfortunately, it was mostly downhill! I don't consider that a true average!