Every rider is going to be different... I usually average 14 -16 mph on my urban commute and that comes up to 18-20 mph on my rural commute because I have 25 miles where I don't see a stop light and can maintain a nice cruising speed.

My old commute was 8 miles and the fastest I ever did this was in 22 minutes... at the time I was in training mode and was treating every ride like a race and had a very straight route with very few stops.