The areas shaded in yellow are dense urban areas where roads, of course, are very near. The areas shaded in green are places where the average distance to the nearest road is 20 kilometers (~12.5 miles) or more. It's not surprising that Montana is very green. The most remote portions of wild Montana? The Bob Marshall Wilderness, Glacier National Park, and portions of Yellowstone National Park. Also very remote; the land south of Fort Peck Lake. I can personally vouch for that as I have explored some there trying to make Montana photos.
Surprisingly, California seems to have an abundance of remote areas. Nevada, Idaho, Utah, and Arizona are also quite remote, it seems.