Whew! I'm glad I read this post! I was worried because I'm only getting 16.5 - 17 mpg, but now I don't feel so bad. Like the OP, I have a short commute at low speeds (5.5 miles, 20 mph avg speed) with plenty of idling.
I was apprehensive because my previous car, an '05 Honda S2000, got 20 mpg during the same commute.