I have a short 6 mile commute. Its downhill then uphill the whole way (effectively theres a 'dip' in between me and work) whether I'm coming or going. I record my runs on GPS, which doesn't adjust for time spent at lights/held up.

I tend to manage 16-17mph average going into work, and 17-18 coming back home. Unless its a night shift, i. which case its a tired 15mph ish!