Depending on how you measure, as far as 250 miles.
There are parts of the United States where the Interstate Highway System does not go. This concept is somewhat foreign here on the East Coast, where the IHS is spun as tightly as a spider web and sprinkled with urban hubs. It's hard to plan a journey of a few hundred miles in the Eastern U.S. without traveling on an Interstate. If you play this game (people do), you'll find that the longest routes that don't involve our biggest highways are all in the West, where the system looks more like a soccer net, leaving wide swaths of land untouched. Somewhere in one of these dead zones of the world's biggest public works project lies, at least by one measure, the most isolated town in the United States.
By all accounts, this distinction is not particularly meaningful. Interstates are not anti-isolation machines: there are big cities without Interstate Highways, like Fresno, California, and Brownsville, Texas, and there are stretches of Interstate with no humans in sight. There are a number of more statistically rigorous ways to calculate isolation. Some of these are fun: Gorda, California, on the cliff-hugging curves of Route 1, is legendary for having the highest gas prices in the country.
But no method has the visual appeal and charming simplicity of distance from the Interstate Highway System. For record-keeping, trivia, and wonky U.S. knowledge, it is a relevant question: what town in the United States is farthest from an Interstate Highway?
As far as I know, none has tried to claim the mantle. The obvious winner is Barrow, Alaska, which can't be reached by road at all. It's 500 miles from Fairbanks, which itself is 600 miles from the state capital in Juneau. But limited to the Lower 48, the answer is disputed even among people who bother to figure this sort of thing out. You may want to open Google Maps and follow along.
Earl Swift, the author of Big Roads, has ventured a guess. He concluded in a footnote to his history of the Interstate Highway System that the furthest town from the Interstate was Morgan, Montana, a tiny outpost of a few houses on the Canadian border. Northeast Montana is one of the great empty stretches of the United States, and Swift calculated that Morgan is 183 miles in a direct line from I-15.
John Nelson did a similar calculation, but came up with Whitewater, Montana, which is 175 miles from Glendive and I-94. Nelson must have discounted Morgan, based on its questionable status as a "place," in favor of Whitewater, which has a population of 64. Nelson concludes that the top seven farthest towns from the Interstate are all in Big Sky country or in the Nevada desert.
Indeed, diving into the polygons of the Interstate map, the Nevada desert is a promisingly blank location. If you colored in the map so that Interstate Highways marked the borders between states, the Nevada desert would be one of the biggest states in the nation. There are no Interstate Highways in the triangle of about 100,000 square miles between Los Angeles, Sacramento, and Salt Lake City. The area between the big roads here is some of the most forbidding territory in the world, home to "the Loneliest Highway in America," (U.S. Route 50).
If you set a minimum population marker at, say, a thousand, your winner might be the old Gold Rush town of Tonopah, Nevada, which is 150 miles from the Interstate but has a population of about 2,500 people, about 20 times the size of most other towns this distance from the network.
If you wanted to use population rather than distance as a priority, you could take this quiz and try to figure out the biggest cities not on an Interstate Highway. (Hint: most of them are in California.)
But calculating miles from an Interstate Highway "as the crow flies" seems like an odd choice -- after all, this is about driving, not flying. How many miles away are these towns by road?
Using driving miles -- by which distances are substantially longer -- and setting a minimum population of 20,000, geographer Brandt Maxwell came up with this list, which, in the continental United States, proclaims Key West the farthest city from the Interstate.
Insisting merely on any evidence of a town at all, and measuring in driving miles, I got a surprise result: after scouring the Great Plains (Hugoton, Kansas: 156 driving miles from the Interstate) and the Western deserts (Topinah, Nevada: 203), I found that the farthest places from the Interstate are actually east of the Mississippi, on a peninsula that juts north into Lake Superior from Upper Michigan.
On the northern edge of this spit are two neighboring towns -- Copper Harbor and Eagle Harbor, which are, respectively, 251 and 238 driving miles from Interstate 39 in Rothschild, Wisconsin. (By comparison, Morgan is 244, and Whitewater, Tonopoh, and the others fewer still.) I believe they are the farthest towns in the continental United States from an Interstate Highway, and they look the part, like an artist's sketches for Hemingway's stories of the Upper Peninsula. Eagle Harbor has a population of 76, Copper Harbor 108. (In fact, even if you measure miles "as the crow flies," Eagle Harbor narrowly beats out Morgan, Montana, for the title -- it is 190 direct-line miles from the nearest Interstate Highway to Morgan's 183.)
An honorable mention is due to Paisley, an incorporated town in the southern half of Oregon near the California border. Unlike the isolated towns of Montana and Michigan, Paisley’s separation is not enforced by the Canadian border. And unlike Eagle Harbor, which is a township, Paisley, population 243, is incorporated. The quickest route to the Interstate from Paisley is 209 driving miles, northwest toward Eugene, which is by most standards a formidable distance.
But the route to the nearest Interstate, due west toward I-5 in Ashland, is a paltry 162 miles. To travel those 162 miles takes an astounding four hours and 48 minutes, which is about as long as it takes to get from Copper Harbor to the nearest Interstate in Rothschild. For a long time, heading west out of Paisley, you must roll down a National Forest Road through the Freemont National Forest. Like the Upper Michigan towns, Paisley isn't a ghost town border outpost -- it has an annual Mosquito Festival, an active community theater group, and occasionally a handful of international exchange students. But if this isn't the middle of nowhere, I don't know what is.
Top image: Flickr user Daily Invention.