My question is this is the average speed of what?

Average speed of walking the dog by bicycle along the neighborhood MUP?

Average speed of a solo Century ride?

For me, my cycling fitness probably peaked in my early 50's when I spent a few years "car-free". I can't say about my actual speeds. I'm not a fast rider.

In High School, I used to ride with the traffic some. There was about a 10 block stretch in downtown Springfield with lights timed at about 20 MPH. It was tough in High School. I started hitting that stretch of road in my early 50's, and got to the point where the lights weren't a problem, but the slow cars kept getting in the way.