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Primary interstates are the major interstate highways of the United States and are assigned a one or two-digit route number. Even route numbers are assigned to east/west routes, with the lower numbered routes being further south (I-10) and higher numbered routes in the north (I-90). Similarly, odd route numbers are assigned to north/south routes, with the lower numbered routes being further west (I-5) and the higher numbered routes being further east (I-95).
Three-digit interstates are spur or loop interstates that usually service large cities and suburbs. The last two digits of a three-digit interstate are always the number of the parent route, the primary interstate where it originated. If the first digit is odd, the interstate is typically a spur route while if the first digit is even, the interstate is typically a loop route. Three-digit interstates are covered in their parent route's article.
The interstate highways in Hawaii are signed with the standard Interstate shield, with the letter "H-" prefixed before the number. They are fully controlled-access routes built to the same standards as the mainland Interstate routes.
The Federal Highway Administration funds four routes in Alaska and three routes in Puerto Rico under the same program as the rest of the Interstate Highway system. However, these routes are not required to meet the same standards as the mainland routes:
"Highways on the Interstate System in Alaska and Puerto Rico shall be designed in accordance with such geometric and construction standards as are adequate for current and probable future traffic demands and the needs of the locality of the highway." [1]
Alaska's "interstates" are signed as state routes -- and the state route numbers do not coincide with their official interstate designation [2].
Like Alaska, Puerto Rico signs its "interstates" as territorial routes, and the numbers do not match their official interstate designations. However, many of the territory's routes are access-controlled toll roads. [3]
