This might be news to some LA natives, but referring to freeways with the definite article “the” is a thing that’s unique to Southern Californians. Over at KCET, Nathan Masters says it all started because the region was an early adopter of the freeway.
In SoCal, routes were were given names as they opened, and the names corresponded to where the freeways ended or places they passed through—like the Arroyo Seco Parkway or the Hollywood Freeway.
The routes also had numbers assigned to them, but it wasn’t uncommon for freeways in the LA area to have more than one route number attached to them. That meant that, for a time, the Pasadena Freeway was Route 6, 66, and 99.
Apparently, keeping track of all those numbers was as confusing as it seems, so people started to favor destination-based names of freeways. So the freeway that people took to get to San Bernardino was referred to as the San Bernardino Freeway instead of a jumble of numbers.