I've lived in the West all my life (California, Washington, Oregon, Idaho, Montana, Utah, Arizona), and have never traveled farther east than the high plains of the Rocky Mountain Front and West Texas. So it has always seemed to me that there are a huge number of lightly traveled roads here, especially in farmland like the Sacramento Valley, San Joaquin Valley, Willamette Valley, Eastern Washington, and the vast areas of wide open sageland and desert. But then I've had nothing to compare it to, and have never experienced Western roads from the perspective of a bike tourist.
The West certainly has a lot more mountains and fewer towns (although way too many towns by typical Western preferences!), and those two factors would have to narrow and lessen the quantity of travel routes and thus create relative congestion. Are roads in the Appalachians and Adirondacks similarly congested?