What I found out about commuting speed is that it depends on how long you want the ride to be. Here is what I mean by this.

When I was riding 17.5 miles every day to work and back. I was getting a 19 to 21 mph avg speed. 48 to 58 min or so each way. Then started riding 12 miles and my time was 45 min each way. 16 mph avg speed. Then it was 10.2 miles each way and did it in 42 min avg. About 15 mph avg speed.

See I want the ride to last between 30 min to 1 hour. So I adjust my speed to match that ride time.