Cruising speed 19-21mph, a bit faster on a good day. I ride a Surly Cross Check with 25mm GP4000 tires. Tires and bike type can be a huge factor in speed. Even though I do not use the drop position of my handle bars much my average speed went up 2 mph over my mountain bike running high pressure road tires.
My computer tells me my average speed is 16 mph
My commute route varies from day to day, anywhere from 5.5 to 10 or more miles one way, depending on the route for the day. Generally between 7 and 8 miles one way.
Most of my commute routes are fairly flat, with a minimum of stop lights or stop signs. That is why I take those routes, I would rather spend more time pedaling then sitting at intersections.