Marshall County, Iowa.

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.