San Diego. The I-5 and the I-15 head north, and the I-8 heads east; these are the lowest numbers in the system, excluding Hawaii.

Postscript March 3, 1998.
My answer isn't really correct. Alert reader (to quote Dave Berry) Edward Frederick pointed out that the I-4 runs through central Florida, connecting Tampa, Orlando, and Daytona Beach. I-4 is the lowest number in the system. However, it doesn't run farther west than Tampa (couldn't, of course, given the Gulf!), and so is a kind of "pathological case" that messes up an otherwise perfectly cool trivia question. Ah well...