As constructed in the 1920s, State Highway 91 went from Leadville, via Climax, Fremont Pass, Frisco, and Loveland Pass, to Empire, where it joined US 40. The segment from Leadville to Climax was paved by 1936, and the entire route was paved by 1954. In 1938, route 91 became US 6, until US 6 was rerouted over Vail Pass in 1941, leaving the portion of route 91 from Copper Mountain (formerly Wheeler Junction) to Leadville as the surviving part of this historic highway.