$500 billion (2008 dollars)
Holding the distinction of being the largest in the world, the Interstate Highway System is a coast to coast network of state-owned roads. It is a system of high-speed highways that serves every major city in the United States for the transportation of products and goods, personal travel, and for business.
Original Conception - 1956
Conceived during the Eisenhower administration in 1956, the concept was adopted through the strong efforts of the President to meet the challenges of a more mobile society. As an extra benefit to the country, the sprawling web of superhighways would not only provide a more effective method for commercial and private transportation, but also had the potential for providing more avenues of mobility for any military contingency, as well as the means for evacuating major cities in emergencies and national disasters.
There are two categories of Interstates, the primary Interstates and the auxiliary Interstates. Over the years we’ve become quite familiar with the numbered red, white, and blue shields that identify each of the Interstate Highways. The numbering system was adopted in 1957 and revised in 1973 by the American Association of State Highway and Transportation Officials (AASHTO). The association mandated that all Interstate Highways would be identified with either a two or a three-digit number. However, the two-digit number could only be used on Interstates designated as “primary” Interstate Highways. And to be more specific, the two-digit number had to be an even number if the Interstate was an east to west highway such as I-90, or an odd number if it was a North to South primary highway such as I-95.
Complex Naming Schemes
If that wasn’t enough, of all the primary Interstates that carry an odd or even two-digit number, if the Interstate was also classified as a major artery that carried vehicular traffic long distances, it had to be identified with a number divisible by five. Some states have used Interstate signs that also included the highway’s heading, for example I-00E and I-00W. Although AASHTO ruled against such markings, some of them still remain.
The three-digit number is allowed for used on auxiliary Interstates. These are the so-called circumferential highways and spur highways located primarily in urban areas.
These auxiliary Interstates always branch off from a “parent” Interstate Highway. Although there are exceptions, in most cases the three-digit number must consist of a single digit prefixed to the number of the primary Interstate nearby, such as I-495. However, if the auxiliary Interstate returns to its “parent”, it is called a circumferential Interstate and is given an even first digit. If the Interstate does not return to its parent, it is called a spur Interstate and given an odd first digit. Despite the ban imposed on primary Interstates signs, however, auxiliary Interstates are allowed to use north/south, east/west identifications.