What is the longest single-numbered highway in the United States?

The longest single-numbered highway in the United States is U.S. Route 20.

US 20 stretches about 3,254 miles from Newport, Oregon, on the Pacific coast to Boston, Massachusetts, in New England. It crosses 12 states and is part of the United States Numbered Highway System established in 1926. Its number ends in zero because it is classified as a major east–west route.

Although the highway is commonly described as coast-to-coast, its official designation is discontinuous through Yellowstone National Park. Unnumbered park roads connect the western and eastern portions, so the route is not one uninterrupted numbered pavement corridor. US 20 was originally shorter and ended near Yellowstone’s eastern entrance before being extended westward in 1940.

Travelers often confuse US 20 with Interstate 90. I-90 is the longest Interstate Highway, while US 20 is the longest U.S. Highway. US 66 is more famous historically and culturally, but it is much shorter and was decommissioned in 1985. US 1 is also a major route, yet it runs along the eastern edge of the country rather than spanning the nation’s widest road distance.