A more realistic speed for commuters is about 15-18 mph which should be relatively easy to maintain in fair weather conditions and a well conditioned aerobic system. Since it's not everyone's goal to work out your anaerobic system; around my alley only the few speedy ones get about 18 - 22 mph a couple these fellows seem bike messengers. Anything above 20 requires a significant amount of effort to be put out regardless of bike type.

When I started to commute this past Spring I was barely able to keep about 14 mph. I had to veer off the road to be really sick one morning the first week & I thought was on good shape. Nearly 3000 miles later I can say that I am one of the fast ones, quitted smoking and lost about 10 pounds ( I still drink beer) just last week I had that chase game on nearly perfect conditions for about 10 miles, during the last stretches we we furiously pedaling at 28 mps, later at the office I saw to my amazement the we peek at 31!
Hanging in there for dear life last minutes my only thought was 'cmon man just give it up! while fueling more on pride than glycogen and oxygen I knew that I had only few more seconds feeling that my lungs were on the border of collapsing the guy shifted direction and rocketed for a ped bridge access to cross Michigan Ave.

I like to think that he stopped to catch his breath under the bridge, while I decelerated and continue riding next to the raising sun. I felt robbed from the bragging rights to have passed a beautiful racy yellow road bike while commuting on my humble hybrid Trek.

"There's no such a thing as tough, just trained and untrained, which one are you? " D. Washington