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From Wikipedia, the free encyclopedia
Primary Interstate Highways are the major interstate highways of the United States and have a one or two-digit route number. Even (2, 4, 6, 8, or 0) route numbers are given to east/west routes, with the smaller numbered routes in the south (I-10) and bigger numbered routes in the north (I-90). Similarly, odd (1, 3, 5, 7, or 9) route numbers are given to north/south routes, with the smaller numbered routes in the west (I-5) and the bigger numbered routes in the east (I-95).
There are five sets of numbers that are used more than once throughout the system; the highways whose numbers are used more than once are separated by big distances to avoid confusion. Below, these are made separate from each other by West and East.
Three-digit Interstates are spur or loop Interstates that usually are in big cities and areas outside of cities. The last two digits of a three-digit Interstate are always the number of the route it is related to. If the first digit is odd (1, 3, 5, 7, or 9), the Interstate is usually a spur route while if the first digit is even (2, 4, 6, 8, or 0), the Interstate is usually a loop route.