It was in 1956 that President Eisenhower signed a bill establishing the Interstate Highway System. Today, that system is America’s main street — linking all 48 continental states along its more than 46,000 miles, and making it the largest highway system in the world.The original cost of the interstate system was more than $33 billion — at the time, the largest public works project in the nation’s history. From rural dirt roads to the interstate highways, there are 4 million miles of roads in the U.S. Building new roads and maintaining existing ones costs $153 billion a year.