Learn the logic behind the US interstate numbers

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The numbers on U.S. interstate highway signs may seem random, but they can actually be an important navigation tool. This short video shows how it all works.

For starters, the interstate system is divided into majors, mediums, and minors. East-west majors get double-digit numbers ending in zero, with higher numbers for the more northern routes. That’s why the northernmost interstate (connecting Boston and Seattle) is I-90, while the southernmost (connecting Jacksonville, Florida, and Santa Monica, California) is I-10.

Interstates aren’t always sequentially numbered, however. I-50 and I-60 don’t exist because those numbers had already been used by highways that were part of the pre-interstate system (which also birthed Route 66).

The north-south interstates also get double-digit numbers, but this time ending in “5” to indicate their direction, and lower numbers for the routes furthest west. That’s how the interstate connecting Maine and Florida became I-95. The route connecting California and Washington State is called I-5, a single-digit number, but still appropriate for this most-westerly route.

Interstate minors diverge from the majors and get three-digit numbers. The last two digits match the major they diverged from. If the first digit is even, that means the minor will eventually reconnect with that major. For example, I-670 splits off from I-70 in Kansas City, Missouri, and reconnects in Kansas City, Kansas.

If the first digit in a minor-route number is odd, the highway doesn’t reconnect with the interstate it started from. That’s why these highways are sometimes called spurs, although they can also connect with a different interstate. Because this system only allows for limited number choices (1-9 plus the parent highway’s number), there are many duplicate minors. Both Maryland and Massachusetts have an I-495, for example.

Finally, interstate mediums use double-digit numbers, with even numbers indicating east-west routes and odd numbers for north-south routes. However, some of the numbers are duplicated. That’s allowed as long as the routes in question are far enough apart to avoid confusion.

So while there are some exceptions, it is possible to look at an interstate number and figure out which direction you’re traveling, and what major highways the smaller interstates connect to. Keep that in mind during your next road trip.

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