The approximately 0.09 miles of CA 225 that still exists as a rail underpass.
In that vein..the portion of Route 275 that still exists (just the Tower Bridge and nothing else of the route)!
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Since Route 18 was already mentioned, another set of California examples:
- the north-south part of Route 84, which navigationally doesn't really jive with the more well known east-west portion (even if Vasco Road corridor someday was actually signed as 84 to connect the two segments)
- Route 238 even if the I-238 segment was still state route only - the "bump" with 580 means that those two segments really do not operate as one cohesive route at all. If anything, the state route portion of 238 really should be tied together with 185 instead.
- Route 14U
- Route 121 (which pre-1964 was the original Route 37 past Sears Point) - essentially, direct north then direct east with a concurrency, then direct north with another concurrency, then direct east, before heading north/northeast. A zig-zag of a route from 37 all the way to the end at Route 128.
- L-shaped corridors like Route 193 and 175, and to a degree Route 70, where it is extremely unlikely one would realistically try to follow the designation end-to-end.