An Explanation of the US Interstate Numbering System
From CGP Grey, here’s an explanation of the numbering system used by the US Interstate Highway System. Here’s the basic deal, from Wikipedia:
Primary Interstates are assigned one- or two-digit numbers, while shorter routes (such as spurs, loops, and short connecting roads) are assigned three-digit numbers where the last two digits match the parent route (thus, I-294 is a loop that connects at both ends to I-94, while I-787 is a short spur route attached to I-87). In the numbering scheme for the primary routes, east-west highways are assigned even numbers and north-south highways are assigned odd numbers. Odd route numbers increase from west to east, and even-numbered routes increase from south to north (to avoid confusion with the U.S. Highways, which increase from east to west and north to south).
In-car and on-phone GPS systems have made knowing this system largely irrelevant for most drivers. I spent a lot of time in the car as a kid — summer roadtrips around the country and frequent local travel out of our rural area — and loved maps & atlases even at that age, so this was pure nostalgia for me. The video covers some of the numbering exceptions at the end (like the 35E/35W split in the Twin Cities I used to drive on often), but I would easily have sat through 10 more minutes of them.
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