Linguistically, I believe that the Californian usage (e.g., "the 5") is an elliptical construction -- i.e., it's a truncated form of "the 5 freeway".
Btw. It's nice if you omit the word "linguistically". It's good to remember that just because you speak a language, that's not the same as being an expert on language science. Any more than because you inhabit a body, you're an expert on physics.