Flat rides (30-70 mi) with maybe 600 feet of "climbing" (overpasses) are around 18-19mph average solo or in a small group, 21mph in a big group.

Lightly rolling rides, like my weekly 50 miler, are something in the 3000-4000 vertical feet range, averaging around 16mph for "slow," 17-17.5mph solo, and 17-18.5 in a small to mid sized group.

"Hillier" rolling rides (steeper grades and lots of little climbs with a few longish ones that take it out of me quicker than what I consider lightly rolling), may be more like 30-48 mi and closer to 3000 ft than 3500 or 4000. Average speeds for me (typically solo on those roads) are around 15.5-17.5mph.

Then there are mountain rides of 45-60 miles (in a group technically, but many of the miles are too difficult for me to keep up with the guys enough to get much drafting benefit, and I'm also slower on descents). 4500-7000 feet of climbing. Average speeds from about 15-17mph (the 17mph ride involved a good sustained stretch of a rotating paceline around 20mph).

I have definitely gotten faster on certain terrain in the 2 years I've been cycling. These are my more recent numbers.