On a road bike, with favourable traffic lights, going all out the whole way, I've been able to do my 9.6 mile commute in about 35 minutes. That's mostly flat, but I have one good downhill section, and one good uphill section (start and end elevations are very close). Like others here, I can ride 20 mph easily enough, but 20 mph average in the city is pretty difficult, at least for me.