I had no idea about the interstate numbering system. This is awesome to know, thanks.

It's a cool system. The numeric rules are (though there are lots of exceptions):

* Primary interstates are one or two digits. Shorter spurs, loops, and connectors are three-digits.

* East-west primary interstates are even-numbered, north-south are odd.

* Major arterials are divisible by five. (The longest interstates are I-90, I-80, I-40, I-10, I-70, I-95, and I-75.)

* Odd routes increase in number going from west to east.

* Even routes increase in number from south to north.

* The last two digits of a three-digit interstate usually identify its main parent interstate. (For example, the I-310 spur connects I-10 to US 90.

Even routes increase from south to north (e.g. I-10 runs through Arizona, I-90 runs through Washington).

Oops, fixed.

Another fun tidbit is that any 3-digit interstate is an auxiliary route of the 2/1-digit mainline. The first digit determines the type of route, with odd being a spur and even being a bypass or beltway. So I-405 is a bypass of I-5, and I-110 is a spur off I-10 connecting it to the Port of LA.

I-110 is also a spur connecting I-10 to downtown Pensacola, FL, Biloxi, MS, and downtown Baton Rouge, LA.

And apparently there's a I-405 in Seattle. (All the I-110 spurs I've driven on.)

> And okay, generally East/West interstates use even numbers representing what % of the country lies to the South while North/South use odd numbers representing what % lies to the West.

The idea that the highways are designed and numbered to represent the proportion of land area of the contiguous US seems specious. Source?

Edit: the interstate highway going north from Kansas City is 29, but obviously much more than 29% of the US is west of I-29.

The semantics aren't exactly meant to be "% of land area", but they do represent a rough coordinate system[1], from (5, 8) in the SW corner of the contiguous US (San Diego) up to (95, 90) in Boston.

Interestingly, US highways follow the opposite system[2], going from (1, 2) in the NE corner of the country (Maine) to (former) (101, 80) in San Diego.

[1] https://en.wikipedia.org/wiki/Interstate_Highway_System#Numb...

[2] https://en.wikipedia.org/wiki/United_States_Numbered_Highway...

Those us routes came first and the interstates are purposely numbered backwards of it to avoid confusion.

Numbering from west to East and south to north and using evens and odds to mean East-west and north-south makes sense.

But that doesn’t imply anything about proportion of country, however the measurement is defined.

Sure it's anecdotal, I just heard it from my parents as a kid.

And obviously it's not exact, just look at how close I-94/I-90 are around North Dakota and compare it to I-90/I-80 or I-80/I-70. And a lot of the "East-West" interstates go diagonally or even North-South in places.

But I doubt they made the roads by laying rulers across the map (except maybe in Kansas or Oklahoma), and it does seem like they generally increment from 0-100 in each direction with odd/evenness decided by the primary axis.

Wikipedia has a lot to say on the matter, but I got bored reading all the special cases like how they number loops that connect two highways: https://en.wikipedia.org/wiki/Interstate_Highway_System#Numb...

Thank you I had no idea how the numbers were chosen until now!