Why makes highways in California so effin' special?

They’re all referred to with the definite article THE.

Like the 405 or the 101… they’re almost universally preceded by THE by Californians. And when I’m there, I find myself doing it, too.

But then, elsewhere in the country, no one seems to refer to I-40 as “the 40” even though it too runs across California. Matter of fact, the California transplants I know don’t use THE when referring to highways in other states either.

Or maybe it’s a regional thing in Cali? Do folks in San Francisco, for example, use a definite article when referring to a highway?