Bikes that are marketed as 'commuter bikes' are just that...marketing. As of today, I have ridden 2886 days to work since 1/1/88. I rode back and forth to school and work for several years before that but I have no records (pre-bicycle computers). 59% of all those rides have been on mountain bikes and 41% have been on road bikes. Not a single trip has been done on anything that resembles the 'commuter bike' that is marketed today because they have only been marketed for a few years now. I suspect that most commuting cyclists who have been doing this for any length of time (in the US) haven't used a 'commuter bike' because they are a recent offering. I suspect that most commuting cyclist have a bike they commute on. They may even call it a 'commuter bike'. Some of them may even resemble the bikes that are being marketed as 'commuter bikes' but most of commuting cyclist are going to a bike of many, many different models, varieties and disciplines.

Hitting 20 mph may not be a necessity for you. As a 'business professional', I hit speeds far in excess of 20mph all the time while commuting. I can hit 20 mph in the first hundred yards on the way home from the lab and often hit in excess of 40 mph by the time I reach the access gate which is about 1/2 mile away. If the wind is blowing, I can exceed 40 mph without even trying and maintain 30+ mph for a significant portion of the ride home. And I don't compete. I just like to go fast.

And there are times when 20+ mph is a necessity on my commutes. Crossing intersections and keeping up with traffic before the road pinches down is one such place. If I jump on the pedals, I can be up to 30 mph before the first block and I can control the flow of traffic around me so that they don't squeeze me up against the curb.

Commuting, like all bicycling, depends on the rider...not on any set rules.