Highway Linguistics

HIGHWAY LINGUISTICS....Last month, when he was in town, I had dinner with Matt Yglesias, and when we were about done we got to talking about directions back to his hotel (this was a few minutes before I got lost taking him there). He noted that I, like other Californians, refer to freeways using the definite article: "the 5," "the 405," "the 10," etc. Back east, I guess, you don't do this, do you? It's — what? "Highway 5"? Or just no identifier at all, as in "Take 10 west until you fall into the ocean and you're there"?

Or what? The odd thing is that the definite article habit isn't quite universal in California, though in my experience it's pretty close. Highway 99, for example, usually seems to be referred to as "highway 99." Ditto for the famously scenic Highway 1. Other state highways vary.

Anyway, this is apropos of nothing in particular. Just curious. How do you do it in your state? And does anyone happen to know where the linguistic variation comes from?