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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

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.

GFK_of_xmaspast 10y ago
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.
- Sanddancer 10y ago
  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.
  - GFK_of_xmaspast 10y ago
    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.
- amyjess 10y ago
  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.
whoopdedo 10y ago
Same thing happened to US-74 in North Carolina. Parts of it are I-74 despite being south of I-40.

GFK_of_xmaspast 10y ago

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.

Sanddancer 10y ago
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.
- GFK_of_xmaspast 10y ago
  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.
amyjess 10y ago
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.

whoopdedo 10y ago

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