Something we learn in perusing this beautiful map of the great American travel fiction: no one has ever written a great road novel about passing through Arkansas. That's an artifact of how the genre is constructed: Lonesome Dove has a story arc that starts there, but it was excluded as apparently too much a work of fiction and too little a fictionalized account of an actual journey that the author took (e.g. On the Road, which was included and certainly could not be omitted from the genre).
Also, apropos of the last post, it doesn't appear that any of them are about Route 66, "The Mother Road." The iconic "Chicago to Los Angeles" route has apparently never prompted a great travel novel of this particular genre. The Grapes of Wrath is, I suppose, like Lonesome Dove too removed from the parameters of the genre. But I'd have expected one from the glory years of the Mother Road, when Bugs Bunny could joke about 'taking a left turn at Albuquerque' and Snoopy could have a brother in Needles and everyone reading a newspaper from coast to coast would know what they were talking about.