What Do The Interstate Highway Numbers Mean?

US Intersate Highway SystemWhile the numbering system for the Interstate Highway System might look like it was chosen randomly, if you look at a national map you’ll quickly see a pattern to how the Government chooses the Interstate numbers.  The American Association of State Highway and Transportation Officials established the following rules when numbering our Interstates:

  • Direction
    • Even numbered interstates travel west to east.
    • Odd numbered interstates travel south to north.
    • Interstate numbering starts in the west/south and increases as you cross the country to form a “grid”.
    • Why?  Because the government didn’t want the numbers to conflict with the (then) currentU.S. Highway numbering system for highways that worked the exact opposite direction.  In theory no US Highways or Interstates would ever exist in the same state that way.
  • Significance
    • Routes ending in “0” are major west-east routes.  (I-80)
    • Routes ending in “5” are major south-north routes.  (I-75)
    • “Primary” Interstates have one or two numbers.
    • “Auxiliary” Interstates are loop or spur highways that principally service urban areas.  They have a single digit prefix attached to the two numbers matching a nearby “primary” interstate.  I-294 in Chicago for example which acts as an auxiliary Interstate in I-94 .
    • Loop routes (and through routes that usually connect to another Interstate) generally begin with an even number (I-465 in Indiana).
    • “Spur” routes that usually do not connect to other Interstates at both ends begin with an odd number (I-196)
  • Trivia
    • I-24 and U.S. 24 in Illinoisis the only exception to the plan of not having highway and Interstate numbers match in the same state.
    • 50 and 60 were not assigned so they wouldn’t conflict like I-24 in Illinois.
    • I-275 that circles Cincinnati is the only Interstate Loop that also travels through (and services) three states.
    • Four State Capitals are not connected to an Interstate: Juneau,Alaska;Dover,Delaware;Jefferson City,Missouri; and Pierre, South Dakota.
    • The legend that the government requires one mile out of every five on the interstate to be straight for emergency aircraft landings is just that – A legend that is not true.
    • Shortest labeled Interstate?  I-375 in Detroit.
    • Longest West/East Interstate?  I-90
    • Longest South/North Interstate?  I-95
    • Heaviest traffic?  I-405 in Los Angeles.
    • Slowest Traffic?  I-95 in Maine.
    • Hawaii and Alaska are exempt from the numbering system since their highways are completely detached from the rest of the U.S.

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