A humorous explanation of America’s often confusing highway numbering conventions

In a rather humorous animated video, CGP Gray explains, in his own quite candid way, how US Interstate Highway numbering conventions work, noting what confusing exceptions to these rules exist today.

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

Gray extends his point by naming the Interstate Majors – highways that cross several states in an east-west direction (two-digit number ending in 0) and in a north-south direction (double-digit number ending in 5).

Together, the Interstate Majors constellation with its glowing numerals lets you navigate the land. The lower the numbers, the further south and west you are. Highest, North and East. If it ends in zero, you are traveling horizontally. And if it ends in five, vertically.

It also talks about interstate (and intrastate) minors (three-digit numbers that derive from the middle finger), Spurs, and interstate mediums.

Interstate minors diverge by a major, and their last two digits match the major they came from. Then, if the first digit is even, that means the Interstate Minor will eventually reconnect to its major parent. … Interstate Mediums follow the same numbering scheme as Majors, all double digits, if we keep those leading zeros in our hearts. Evens: East-West. Ratings: North-South.

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

…the I-495 Freeway aka the Long Island Freeway aka the LIE… It’s the spine of the fish of Long Island, which never connects to its parent I-95 because Long Island is an island. But as a one-way spur, it should start with an odd number, not even. And it really should be a double-digit Interstate Medium because it technically doesn’t connect to I-95 at all… A worse exception on the other side in San Francisco is Interstate 238. Which, according to the number , should be a bypass of I-38. But no, the I-38 does not exist. I-238 is a one-way spur connecting two other Interstate Minors: I-580 and I-880.

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