An Explanation of the Often Confusing US Interstate Highway Numbering Conventions

CGP Grey explains, in his rather forthright manner, how US Interstate Highway numbering conventions work, noting which confusing exceptions to these rules exist today.

The Interstate Highway shields hide within them long forgotten knowledge.  As our great ancestors could navigate by the signs in the sky before the creation of the compass, so too before GPS could they navigate by these signs.

Grey expands his point further by naming the Interstate Majors – highways that move across multiple states in both an east to west direction (two-digit number ending in 0) and north to south direction (two-digit number ending in 5).

Together, the Interstate Majors constellation with its shining numbers lets you navigate the nation. The lower the numbers, the more South and West you are. The higher, North and East. If it ends in zero, you’re traveling horizontally. And if it ends in five, vertically.

He also talks about Interstate (and Intrastate) Minors (three-digit numbers that branch off from the Major), Spurs, and Interstate Mediums.

Interstate Minors diverge from a Major and have their last two digits match the Major from whence they came. Then, if the first digit is even, it means the Interstate Minor will eventually connect back to its parent Major. …The Interstate Mediums follow the same numbering pattern as the Majors, all two digits, if we keep those leading zeros in our hearts. Evens: East-West. Odds: North-South.

Of course, there are exceptions that make this convention a bit more confusing. Examples include Interstate 99, Interstate 495, Interstate 238, and numbering duplicates.

…interstate I-495 aka the Long Island Expressway aka the L.I.E. …The is the spinal spur of the Long Island fish, never connecting back to its parent I-95 because Long Island is an island. But as a one-way spur it should start with an odd digit, not an even. And it really should be a two-digit Interstate Medium because it technically doesn’t connect to I-95 at all …A worse exception on the other coast in San Francisco is Interstate 238. Which the number tells you should be a bypass off of I-38. But no, I-38 doesn’t exist. I-238 is a one-way spur connecting two other Interstate Minors: I-580 and I-880.