Not a 20mph guy, but I've noticed road quality makes a big difference on my avg speed.
When I take the smooth roads, I avg 16 or 17 mph for the whole commute (i'm slow). But when I add in a couple old rough roads, then my avg drops a good mph or so.
I imagine, if I lived in a nice area where the roads weren't old and janky, I'd be a bit quicker.