in California it's *the* I-5.
No, it's "The 5" , no I. And that is pretty much an LA thing. Go to the SF bay area and the rest of the state it is simply 5, or 880, or 101, whatever. Now over in Texas it is often the name of the road: LBJ, Airport Freeway, Central Expressway, Inner Loop, etc. Chicago too. In New York, it's a combo... The Garden State Parkway, The Cross Bronx Expressway, etc