Hawaiian Interstates
There are three mainline Interstates in Hawaii, as well as a 3di spur.  The three mainline Interstates are numbered in order in which they were funded and built.  Their signage (H1 and H3 are east-west routes, H2 is north-south) would have been a clear violation of the rules for even- and odd-numbered routes in the continental United States.

H1 is the longest Interstate Highway in Hawaii, going from Makakilo to Kahala.  It is the main route through Honolulu and the only Hawaiian Interstate with a spur route, H201.  Despite its number, H1 is signed as east-west.

H2 goes from Pearl City to Wahiawa.  Despite its number, it is signed as north-south.

H3 goes from Pearl Harbor to the Hawaii Marine Corps Base.  Despite its number, it is signed as east-west, although most residents consider it as a north-south route.

H201 is a 3di spur route off of H1.  It was previously a hidden route for the HI 78 freeway, in part because the Hawaii Department of Transportation feared that there wouldn't be enough room for four characters, even on a 3di Interstate shield.  Finally, around mid-2004, new signs were put up along the freeway, displaying the entire "H201" designation in a very skinny font.  The designation has been in use since 1989.
Interstate 238
Interstate 238 is the biggest violator of Interstate rules.  It was signed as an extension of CA 238 in the Bay Area.  I-238 serves as a link between I-880 and I-580, and it runs from San Lorenzo to Hayward.  There has never been an Interstate 38, and even if there were, it wouldn't go near the Bay Area.  The number 238 was assigned to the freeway because there were no "x80" spur routes available (CA 180 already exists, and 280, 480 and 980 had already been taken).  A better alternative would be to rename the highway I-480, which is now available thanks to the demolition of the hated Embarcadero Freeway.
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