Well... I'm with everyone else it's not a quantifiable stat for the most part. Too many variables. I will say that year in and year out since I started keeping stats my annual avg speed from all that year's ride has always been somewhere in the 17 mph range.
Now for silly fun this year I've been trying to run each of my 20+ road bikes at an average speed of 19 mph for my base 24-30 mile daily route. I've managed it so far with 19 of the bikes, 3 more to get there with if the bad weather will ever stop.
Thread on this silly game here:
The 19mph avg pace game
Now speed based reality for me is that I've only have one ride this year where I've hit a 20 mph avg. Normally I have several of those a year so to me this says I'm slowing down. Riding the same route so much year in and year out means avg speed is an indicator to me about me but nothing more. You get a feel for how the wind and conditions are affecting you on the route as well as knowing what you should be able to to under a given condition such as winds and temperature and such. But it still isn't some comparable to any other rider unless your riding side by side or such.