Map of the United States showing the Interstate Highway System.
The Dwight D. Eisenhower National System of Interstate and Defense Highways (commonly known as the Interstate Highway System or simply the Interstate) is a network of controlled-access highways that forms part of the National Highway System in 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. In 2016, it had a total length of 48,181 miles (77,540 km). As of 2016, 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 $499 billion in 2016).
Interstate sign[]
The Interstate Highway System, indicated by a red and blue shield with white numbers, is a system of entirely freeways (unlike the U.S. Highway System, which is mostly undivided surface roads). The Interstate System is also based on a grid, with east–west routes bearing even numbers and north–south routes bearing odd numbers. In order to prevent confusion with the earlier U.S. Highway System, however, the Interstates are numbered in the opposite direction, such that the lowest route numbers are in the south and west, and the highest numbers in the north and east. Major routes end in either a "0" or a "5"; for example Interstate 10 spans the country from Jacksonville, Florida, to Santa Monica, California, while Interstate 35 goes from the Mexican border to the Great Lakes. Like with U.S. Highways, spur routes are numbered by adding a hundreds digit to the parent route. Because of the large number of spur routes, three-digit numbers may be repeated within the system, but unique to each state. Additionally, the parity of the hundreds digit tells the nature of the spur route: odd hundreds digits like Interstate 393 only connect to the system at one end, while an even hundreds digit like Interstate 440 indicates that the highway connects to another Interstate at both ends.
