San Diego to Arizona border

The entirety of Interstate 8 in California is defined in section 308 of the California Streets and Highways Code as Route 8:

Route 8 is from:
(a) Sunset Cliffs Boulevard to Route 5 in San Diego
(b) Route 5 in San Diego to Yuma via El Centro.

The section of Route 8 defined in subdivision (a) is not considered an Interstate Highway according to the Federal Highway Administration