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 freeways 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. The network has since been extended, and , it had a total length of , making it the world's second longest after China's. , about one-quarter of all vehicle miles driven in the country use the Interstate system. The cost of construction has been estimated at $425 billion (in 2006 dollars).