I think what everyone is saying is that if a car can average > 20 MPH, then a bike theoretically could too.

Lights and stopping can be a problem. But, for example, on one of my rides, there are about 10 blocks of traffic lights, all timed at around 20 MPH. I think they may have been timed at about 25, but they slowed them down to the speed limit. Anyway, if one can hit the first one right, then one can hit all of them... unless one gets messed up in traffic.

Anyway, to get a 20 MPH commute with a reasonably through route, one must simply be able to ride at about 22 MPH... well within the grasp of the average TDF rider.