My commute used to be 8000 km over the course of a year and with some long term data on the computer my average speed in nice weather was 21 kmh in the city (using many bicycles) while my rural commute to the frame shop which has no breaks for 40 km over a 50 km route was 25 kmh which is closer to the speeds I average when I am touring.

My winter speed was not much slower although there would be some days a normal 30 minute commute would turn into a 45 minute commute because of heinous weather conditions.

This is not to say that on the right day with the right bike that I could not cover my 8 mile urban commute in 22 minutes or hit 55 kmh when I was crossing the bridge or cover certain flat sections at 40kmh +.

Put me on the road bike and send me out for 100km in the country and my average speed will be in the low 30's.

My current commute to the bike co-op is 8km and I usually plan on it being a 25 minute ride in traffic although this can vary by quite a bit depending on the time of day, traffic, and luck of the lights. Some days I can make the trip in under 20 minutes and some days it is closer to 30 minutes.