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Comment by pklausler | Hacker News Reader

I-82 in Washington State is a glaring exception to the numbering convention, being located north of I-84. Are there other exceptions?

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There's a fun bit of trivia to that; it's actually I-84 that's the exception.

Back when the Interstate Highway System was created, it was designed with intentional forks in the road that will both go to the same start and end points but get there in different ways. These forks shared the same number but had directional indicator suffixes. One example of this was I-80S and I-80N.

Eventually, AASHTO decided that system was confusing, so they decided to eliminate most directional suffixes and assign unique numbers to each highway. As such, I-80S became I-80, and I-80N became I-84. They would've used I-82, but that highway already existed, and they decided against resigning an existing non-suffixed interstate.

Some directional suffixes continue to exist, but only for interstates that fork for the duration of a single metropolitan area with two principal cities. The best-known example of this is I-35, which forks into I-35E and I-35W twice, once in Dallas-Fort Worth and again in Minneapolis-St. Paul.

Another glaring exception is Interstate 238 in California. It's an important, if short, route in the Bay Area, but at the time it was redesignated an Interstate, every x80 number had been taken, either by other local freeways, or by unrelated highways. So, when it was updated to interstate standards, they just decided to let it keep its old number, even though interstate 38 doesn't even exist.

Since then, Highway 480 (it was never formally adopted as an Interstate) has been removed, but renumbering would be more trouble than it's worth, so it's remained I-238.

There's also I-595 between DC and Annapolis, which is signed as US-50 instead because there are too many x95s in the area.

There's a simpler explanation. It's a pre-existing route that was adopted by the federal highway system. US 50 is well known enough that resigning it would be more trouble than it was worth.

Interstates don't usually spring out of thin air, they're often existing US Highways or state roads that get relabeled or upgraded to interstate standards and relabeled.

Many Interstates share numbering with US Routes. It's a way to keep around some US Routes that share some of their path with an Interstate but then diverge sharply in other areas without having to maintain two roads in parallel

Head over to Google Maps and zoom in on some random Interstates; you'll see the dual numbering. A good example is I-10 and Route 90 in southeast Texas. Everything between roughly Columbus and Vinton (including the whole Houston area), I-10 and Route 90 are the same road, but they go their separate ways on both sides. Or for a north-south example, I-35E and Route 77 in the Dallas area; they're joined between Denton and Waxahachie.

Same thing happened to US-74 in North Carolina. Parts of it are I-74 despite being south of I-40.

I-99 is in the middle of Pennsylvania.

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