The origin gets asked around here occasionally as a trivia question. The road mentioned below back then was very rural. From Wikipedia:

Inspiration for the title line had come while Taffy Nivert and Bill Danoff, who were married, were driving along Clopper Road in Montgomery County, Maryland, to a gathering of Nivert's family in Gaithersburg, with Nivert behind the wheel while Danoff played his guitar. "I just started thinking, country roads, I started thinking of me growing up in western New England and going on all these small roads", Danoff said. "It didn't have anything to do with Maryland or anyplace." ....

Danoff was influenced by friend and West Virginian actor Chris Sarandon and members of a West Virginia commune who attended Danoff's performances. Of the commune members, Danoff remarked, "They brought their dogs and were a very colorful group of folks, but that is how West Virginia began creeping into the song." While the song was inspired by Danoff's upbringing in Springfield, Massachusetts, he "didn't want to write about Massachusetts because [he] didn't think the word was musical."