A Unique Road

[BillyGr]

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.

[AZBuck]

There's absolutely nothing requiring that a three digit Interstate designation be unique, only that it be locally unambiguous. Thus any three digit I-X80 designation used significantly east of San Francisco such as I-180 (PA spur), I-680 (OH spur), I-480 (alternate route south of Cleveland), I-280 (spur into Toledo), I-280 (beltway around the Quad Cities), I-380 (spur from Iowa City), etc., etc., etc. could have been used.

Note that in the above (partial) list I-280 was used in more than one place. Also note that I-680 and the I-280 into Toledo violate another convention of Interstate numbering, that 'spurs' (roads not rejoining the main road) carry odd initial numbers and that 'beltways' (roads that do rejoin the main road) have even first digits.

AZBuck

[glc]

The I-238 number was specifically requested by the State of California so it could match the California Streets and Highways Code

The eastern end connects directly to the north end of CA-238.

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.

They could not use I-180 because there is a CA-180 in the Fresno area. At the time there was already an I-480 in SF, since decommissioned. Their code only allows different types of highways in the state with the same number if they are directly connected.