U.S. Route 40 (US 40) is an east–west United States highway. As with most routes whose numbers end in a zero, US 40 once traversed the entire United States. It is one of the original 1920s U.S. Highways, and its first designated termini were San Francisco, California, and Atlantic City, New Jersey. The western end has been truncated several times, and the route currently ends at I-80 just outside of Park City, Utah.

Starting at its western terminus in Utah, US 40 crosses a total of 12 states, including Colorado, Kansas, Missouri, Illinois, Indiana, Ohio, West Virginia, Pennsylvania, Maryland, Delaware, and New Jersey. One former and four current state capitals lie along the route. For much of its route, it runs parallel to or concurrently with three major Interstate Highways: I-70 from Empire, Colorado to Washington, Pennsylvania and again from Hancock to Baltimore, Maryland; I-68 along the Maryland Panhandle; and I-95 from Baltimore to New Castle, Delaware.

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