I lived in Southern California (Orange County) in the 70s and 80s. We called I-5 the 5. Same for the 405, the 101, the 15, the 10, the 99. Also, we called State Route 1 (aka Highway 1) PCH no matter where it was located in the state, even though SR1 only has the Pacific Coast Highway designation between Oxnard and Dana Point.

I moved to Northern California (Santa Clara County) in the late 80s and I still use the same monikers. If I take a trip to Big Sur, I tell folks I am going to take the 17 to PCH and drive down. Old habits die hard.

https://www.kcet.org/shows/lost-la/the-5-the-101-the-405-why...

“In Southern California, the definite article “the” gets placed before just about every freeway or highway, whereas in the Bay Area just the numbers are said.”

— https://www.kcbx.org/post/how-you-refer-us-101-says-lot-abou...