Gravel
If you are looking for gravel roads, instead of trying to avoid them, here's a couple of links.
The Gravel Cyclist blog had some links in
this post from 2015, including state maps for Colorado, Iowa, and Pennsylvania.
And it pointed me to
gravelmap.com
It shows user submitted gravel roads in yellow. You can bookmark / favorite the view since it changes the URL as you move the map.
About Gravelmap:
Gravelmap is an interactive database of gravel and dirt roads and paths, submitted and managed entirely by people who ride them.
Biking on gravel roads, or "gravel grinding," is becoming increasingly popular. While it may not be a new concept, it is one that is quickly spreading as people discover how awesome it is to explore less-traveled, quiet backroads via bicycle.