Are there any Arizona natives still reading Livejournal?

I have a language question:

If you were talking about the Interstate 10 highway, which one of these would you say:

"There is a terrible traffic accident on I-10."
"There is a terrible traffic accident on the I-10."

Apparently putting 'the' in front of freeway numbers is just a Southern California thing? But I could swear I heard Arizona people doing it as well.