The Federal Aid Highway Act of 1956 appropriated funding for the Interstate Highway System, to construct a vast network of freeways across the country. By 1957, AASHO had decided to assign a new grid to the new routes, to be numbered in the opposite directions as the U.S. Highway grid. Though the Interstate numbers were to supplement, rather than replace, the U.S. Route numbers, in many cases (especially in the west) the US highways were rerouted along the new Interstates.[10] Major decommissioning of former routes began with California's highway renumbering in 1964. The 1985 removal of US 66 is often seen as the end of an era of US highways.[33]
A few major connections not served by Interstate Highways include US 6 from Hartford, Connecticut, to Providence, Rhode Island; US 101 from Los Angeles to San Francisco; and US 93 from Phoenix, Arizona to Las Vegas, Nevada. Three state capitals in the contiguous U.S. are served only by U.S. Routes: Dover, Delaware; Jefferson City, Missouri; and Pierre, South Dakota.
In 1995 the National Highway System was defined to include both the Interstate Highway System and other roads designated as important to the nation's economy, defense, and mobility.
AASHTO is in the process of eliminating all intrastate U.S. Highways less than 300 miles (480 km) in length "as rapidly as the State Highway Department and the Standing Committee on Highways of the American Association of State Highway and Transportation Officials can reach agreement with reference thereto". New additions to the system must serve more than one state and "substantially meet the current AASHTO design standards".[4] A version of this policy has been in place since 1937