It's a California thing.

The joke is that it's a SoCal thing to use the definite article, or so I hear -- although I moved to SF sometime back, never lived in SoCal, and find myself saying "the 101".

I call them the 101 and the 280 as well--and I never lived in SoCal. I think more people do it up here than they'd like to admit. :P

Using 'the' to refer to a freeway is a dead give away you are not a native as is "San Fran" or "Frisco".

However, "the city" and "the bay" are perfectly acceptable.

I could never understand why people cared <_<