Quote Originally Posted by glc View Post
The other one is I-238 in the SF Bay area, it's only a couple miles long and connects I-880 to I-580. There is no parent I-38. The numbering of I-238 does not fit within the usual conventions of existing three-digit auxiliary Interstate Highways, where a single digit is prefixed to the two-digit number of its parent Interstate Highway. The I-238 number was specifically requested by the State of California so it could match the California Streets and Highways Code, and because at the time of the numbering all three-digit combinations of I-80 (the primary two-digit Interstate in the Bay Area) were being used in the state.
Although that is unusual, what would be the option there? If there was no other 80 route to use, it would have to be something odd - either a different number like they did, or going beyond the usual listing to create something like I-1080 (or I suppose using I-080, which might be just as confusing if not in print to see the 0)? Unless any of the other short 80 ending routes could have been merged/split (like where 980 branches from 880, making it 880 west and 880 east or similar like the spurs of the NJ Turnpike), thus freeing the 980 number for this one?

The only other similar issue comes in NY with the 90 prefixes, but at least those are scattered through the state a bit more than these that seem to be quite concentrated.