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. 