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I-790 New York
2.41 miles ; odd L-shaped freeway in Utica; parallels I-90 for 1.5 miles before going south with NY 5 and 12. Dates back to 1965 or earlier.
The road is reportedly barely noticeable these days around Utica as I-790 -- there are a few trailblazers for it, but no I-790 reassurance markers on the highway itself.  Most locals refer to it as Routes 5, 8, and 12, or "the arterial." 
J. P. Wing elaborated on the signing situation :
Furthermore, based on its reference markers, I-790 actually has four ends, not two (ASCII art by J. P. Wing):
| ---+--------- | | |
... so missing signs could be considered a feature.
See a 790 trailblazer photo at J. P. Wing's Photo Gallery.