What average on a roadbike ? Depends what you're measuring. Circuit, one way, etc etc.
The only average i'm interested in is my circuit speed. One way out (usually against the wind) and the return leg (hopefully with a tailwind) combined.
Anything else is meaningless. I can do 24 mph average for 5 or 6 miles on one section of my ride, but its verrrry slightly downhill with a tailwind. I'm fooling myself if i use that to measure against.
Whatever my circuit, hilly, flat, windy...my average is always within one MPH.
I have three routes that take me 25 to 40 miles. I always do 16 to 17 mph overall. It doesnt seem to matter what distance it is.
To those that manage 14 or 15 mph...take heart. At 50, i have to push hard to get 17 mph average.
If i went out 3 times a week, pushed the distances and myself...i could probably get 18 mph average. But i cant be arsed