I tend to go by the Cascade Bicycle Club's ride classification here in Seattle. It assumes average speed you can maintain in the flats. This classification accomodates all classes of riders, so it's a pretty broad range. I believe it to be pretty accurate gauge of what is fast. For my money on a flat ride, if you are going over 20 mph, thats better than average--and depending on the company it can be considered pretty fast.

Easy: Under 10 mph
Leisurely: 10-12 mph
Steady: 12-14 mph
Moderate: 14-16 mph
Brisk: 16-18 mph
Strenuous: 18-21 mph
Super Strenuous: 22+ mph

I trained really hard and rode the Seattle to Portland (STP) in one day two years ago, I got into some really quick pacelines and even with all the stoplights I averaged 20.5 MPH over the first 100 miles (probably 24-25 in those pacelines). I felt destroyed after that--I ended up outrunning all the guys in pacelines (I had four water bottles and was skipping the foodstops that they kept stopping at) and was largely alone (except for a wheelsucker for 10 miles who never said a word to me and refused to pull) and ended up averaging 19 -19.5 for 206 miles. At the end I didn't care how fast I was going I was in total survival mode--it was hot and windy. I remember dropping a water bottle; I did not even think of stopping to pick it up, i was too tired to consider stopping getting off my bike and then starting again--I was on fumes. I started the ride at 144 lbs and at the end, after weighed 138 lbs.