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, it had a total length of 47,856 miles (77,017 km). As of 2013, 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.
Built with the backing of C. E. Wilson who before becoming Eisenhower's secretary of Defense had been the President of General Motors. In his new position he pushed for the development of a national highway system much like the German autobahns.
Not only would this allow American tanks to travel freely if we were invaded by say -- Mexico or Canada (being sarcastic here) but it would stimulate auto sales as people ditched trains and buses and head out on the highway in their Chevrolets. This was very much like what the DuPonts (the family that owned GM, did after WWI) and they wanted to see it happen again but on a grander scale.