I aim for a comfortable 32km/h (20mph) when commuting or biking in general, between stop signs, lights etc. It tends to keep me going just as fast (or faster sometimes) then the traffic when I'm on the road.
I've pushed it up to 37km/h (23mph) a few times, its possible, but only for a short while 10 minutes tops at the moment still relatively out of shape due to not biking all winter like I normally do. Give it a couple more months and I'll maintain that speed as my comfort zone.
Jim