Ah! This is actually a subject I'm perversely interested in. It's only "the 5" in Southern California, where most freeways are commonly referred to as "the #" In Northern California, it's "5" and all freeways are referred to as "#". Hence, "the 405" and "580."

Wow, that was pretty boring, wasn't it?

I am the Doctor, and THIS... is my SPOON!