Interstate Numbering System (idea)

Exceptions to the Interstate Naming Rules

I-238

There is no Interstate 38, but there is an Interstate 238. It runs east-west just south of Oakland, California, connecting I-580 in Castro Valley to I-880 in San Leandro.This 2 mile long portion portition of California State Route 238 was upgraded to Interstate highway status in 1983, but there were no available numbers ending in -80 in the San Francisco Bay Area, so the American Association of State Highway and Transportation Officials (AASHTO) granted an exception to the rules.

Interstate H1, Interstate H2, Interstate H3

These "Interstate" highways on the Hawaiian island of Oahu run the wrong ways by the mainland numbering rules - H1 & H3 run east-west, and H2 runs north-south.