Answer:
The Federal-Aid Highway Act of 1956, popularly known as the National Interstate and Defense Highways Act (Public Law 84-627), was enacted on June 29, 1956, when Dwight D. Eisenhower signed the bill into law.
Appropriating $25 billion for the construction of 41,000 miles (66,000 km) of Interstate Highways over a 20-year period, it was the largest public works project in American history to that point.