Just by looking at the first example you can actually "be more than 18.76 miles from a road" if you don't limit your radius by park boundaries. Probably 20-somthingish just eyeballing the first map without looking at other examples where (maybe) multiple parks share borders.
There's also the assumption that Wilderness Areas are automatically the furthest from roads, machines, and motors. That may be true but I don't automatically take that as automatically true given the size of some of the National Forests out west. It's probably true given the different rules governing Wilderness Areas vs. non-wilderness areas of forests. But I wouldn't make a big bet.
Their definition of wilderness area is quite broad and includes areas within national forests that are "de-facto" wildernesses:
> Four categories of Wilderness area were considered for this analysis:
> - Federal Wilderness Areas, designated by the 1964 Wilderness Act and subsequent congressional action. These are by far the best known and most numerous of the wilderness areas in the USA—there are about 750 of them, preserving federal land managed by the Forest Service, the National Park Service, the Fish and Wildlife Service, and the Bureau of Land Management.
> - “De-facto” Wilderness Areas in National Parks: In most large National Parks (e.g. Olympic, Yosemite, Rocky Mountain), the bulk of the park’s backcountry has been officially designated as wilderness area. However, three large parks—Yellowstone, Grand Teton, and Glacier—have large undeveloped tracts that are managed by park staff as wilderness despite lack of official designation. For this analysis, the large primitive areas of these parks have been included, since park regulations in these areas are essentially the same as in officially-designated areas.
> - State Wilderness Areas: In New York state (Adirondacks and Catskills) and Maine (Baxter State Park on Katahdin), large wilderness areas are managed by the State government. Large parts of these parks have been designated as official state wilderness areas and are managed in a similar fashion to federal wilderness.
> - Tribal Wilderness: Finally, a handful of large Indian Reservations have designated large areas as Tribal wilderness. Perhaps the two most notable are adjacent to federal wilderness in the Wind River Range of Wyoming and the Mission Mountains of Montana. These are included in the overall wilderness areas.
Yeah it really looks like their algorithm is not doing something so fancy as considering the forest roads which in theory are motorable (but may not have seen one in years), so much as measuring from drivable roads and park boundaries which are quite jagged, with significant amounts of park outside the drawn circle... I wonder how much larger the circles would be if you allowed for roughly 1 radian of the circumference to cross outside park boundaries
