varies widely
The best I've done solo for 200 miles is just over 20 mph, but mostly flat. I've done 130 miles at 23 mph with a tailwind. However, most of my riding is in the mountains, and there 18 mph for 60-90 miles or so is really cooking, and frequently it's more like 16 mph average. Long, slow climbs really kill the average. I'll go out at lunch and do 20-30 miles at 18-20 mph, but that's pushing pretty hard. When I did the 508, solo of course, I averaged just under 14 mph, but consider that's with 35,000 feet climbing and temps over 104 degrees, and all stops on the clock.
When I was first doing road riding, I'd be lucky to average 15 mph for 40-50 miles in hilly terrain.
I always report averages in terms of total time, not just rolling time, too.
Doug