My commute is about 15 km, varied terrain, a couple of hills, a couple stop signs, and on average I have to stop at 3 or 4 traffic lights. When I first started commuting last spring, it took about 50 minutes, and I got that down to about 42 minutes by the middle of summer. If I ride my cyclocross bike instead of my regular mountain bike commuter, I'm 5-10% faster. I think the biggest improvement over the year was on the hills. I know I got a little faster on the flats, but I really worked hard on improving my speed up the hills, and I think that's where much of my average speed gain came from.