I simply would be a fool to try to average much more than the 15 or so on my commute - too many chances to gain road rash from the off-angle RR trax, get doored, get head-on wiped by idiot riders going the wrong way, etc, etc, etc.

Commuting, to me, takes too much focus to ride fast like a training ride, I have too many hazards compared with a workout ride out in the country. I can push a bit more on the afternoon return home, due to the route I take home, but still the stretch through town is too dangerous for trying to ride too fast.