All my commutes are on vintage road bikes. Since I have to merge in with rush hour traffic a lot, I have to move quickly. On my 79 Raleigh Competition GS, I have a computer. I have been noticing when I was leaving work, on a slight decline I am pushing 31/33 mph. I got a thrill out moving to the left lane in passing a slower moving car., . Closer to home on a flat section I am up to around 23 mph..
I have to take the full lane a lot of times on my commute, and traffic is moving quite fast.
Most of the time my speed is around 15-18mph, so I could say that is my average speed.