Interstate highways

Dwight D. Eisenhower National System of Interstate and Defense Highways
Highway shields for Interstate 80, Business Loop Interstate 80, and the Eisenhower Interstate System
Interstate Highways in the 48 contiguous states. Alaska, Hawaii, and Puerto Rico also have Interstate Highways. (See version with numbers.)
System information
Length:	47,856 mi (77,017 km)
Formed:	June 29, 1956 (1956-06-29)
Highway names
Interstates:	Interstate X (I-X)
System links
Interstate Highway System Main Auxiliary Suffixed Business

The Dwight D. Eisenhower National System of Interstate and Defense Highways (commonly known as the Interstate Highway System, Interstate Freeway System, Interstate System, or simply the Interstate) is a network of controlled-access highways that forms a part of the National Highway System of the United States. The system is named for President Dwight D. Eisenhower, who championed its formation. Construction was authorized by the Federal Aid Highway Act of 1956, and the original portion was completed 35 years later, although some urban routes were cancelled and never built. The network has since been extended and, as of 2013^[update], it had a total length of 47,856 miles (77,017 km). As of 2013^[update], about one-quarter of all vehicle miles driven in the country use the Interstate system. In 2006, the cost of construction was estimated at about $425 billion (equivalent to $511 billion in 2015).

The United States government's efforts to construct a national network of highways began on an ad hoc basis with the passage of the Federal Aid Road Act of 1916, which provided for $75 million over a five-year period for matching funds to the states for the construction and improvement of highways. The nation's revenue needs associated with World War I prevented any significant implementation of this policy, which expired in 1921.

In December 1918, E.J. Mehren, a civil engineer and the editor of Engineering News-Record, presented his "A Suggested National Highway Policy and Plan" during a gathering of the State Highway Officials and Highway Industries Association at the Congress Hotel in Chicago. In the plan, Mehren proposed a 50,000-mile (80,000 km) system, consisting of five east-west routes and 10 north–south routes. The system would include two percent of all roads and would pass through every state at a cost of $25,000 per mile ($16,000/km), providing commercial as well as military transport benefits.