Sustaining 20 MPH for a short distance-medium disctance can pretty much be accomplished on any bike, but there is a reason most people utilize their AVERAGE speed and not Sustainable.

In Road biking, Averaging 15-17mph is considered "B" pace. This is nothing to shake a stick at especially when doing 30+ miles as most group rides are. 17-19 MPH is considered an "A" pace, at least where I live. Commuting is different, as the distance is usually MUCH shorter and you can choose better routes. But I always was too worried about how sweaty I will be when I get places unless there is a shower I can use.