Exactly. In Chicago the various expressways are just called “I-90”, “I-290”, “I-55”; no “the”. Unless, of course, they’re called, “the Kennedy”, “the Eisenhnower”, or “the Stevenson”.
And in fact, in most of the US, outside of Southern California. Interstate 90 might be “90”, or “I-90”, or “the freeway”, or “the interstate”, or some section of it might have a specific name like “the shoreway” or “the inner belt”, but it’s never “the 90”.
Given that Interstate 90 run east-west from Seattle to Boston, I don’t expect Southern Californians to refer to it very often.
Not quite the same here in my part of Ontario, although it’s not just Californians who put the definite article “the” in front of freeway names.
The convention I’m used to is that ordinary highways which usually have two-digit numbers will be referred to as (for example) “Highway 20”, which may be its official name or may be just a generic term. For more specificity, it might be called something like “County Road 20” or [county name] Road 20. But 4+ lane divided expressways with three-digit 400-series designations will always be referred to as “the 401”, “the 407”, etc. Not exactly sure why this works but it’s probably something about the natural cadence. “The” is also used in front of expressways that have names, like “the QEW”.
In my native land, we had ‘the San Diego freeway’, ‘the Golden State freeway’, ‘the Santa Ana freeway’ (part of the 5), the Pasadena freeway’, ‘the Santa Monica freeway’, and so on. It’s just natural to use ‘the’ ahead of the number.
Coming from Toronto, I’m the same way. It’s “the 401,” “the 404,” and so on. The Don Valley Parkway has no number south of Steeles, so it’s “the DVP.”
And then there’s Lakeshore Boulevard, which is commonly referred to as just, “the Lakeshore.”
Au contraire, mon ami! Must have been awhile since you’ve been here. 
The DVP starts south of the 401. The continuation of the DVP north of the 401 through Sheppard, Finch, Steeles, and beyond is the 404.
Nobody says “the 95” in New England, either. It’s simply “I-95.” Sometimes people omit the “I” and just say “95” but nobody around here would ever add an article.
D’oh! You’re right. Thanks for that.
And I was only just there in 2019, and drove on it. D’oh! again.
Except around Boston where it’s still “128”.
Around NYC there are innumerable named expressways that are all referred to with “the” but no one puts that ahead of any numbered roads. Lots of places have named roads but don’t use that convention, so I don’t think it’s a valid explanation.
Nope, only when they’re writing for a TV show set in a northern city.
And I won’t even attempt to unravel which nomenclature is more “logical” (everyone will think their own local dialect is the natural one), but the point is just that, for whatever reason, it does vary by regional dialect, which often reveals the ignorance of Los Angeles-based writers.
I’m not @Johnny_L.A, but I too grew up out there albeit a few years before he did. I suspect you didn’t quite grok his point.
At one time all those freeways had names but not numbers. So were called “the [whatever name]”. Then all the numbers were added.
Given the established local convention of “the [whatever name]”, it was natural to extend that to “the [whatever number]”. I was a little kid when that happened, so not driving yet. I did notice as I grew up and more new freeways were built and I eventually got old enough to drive myself, that people tended to use “the number” for the new ones over “the name” whereas the older the freeway (or the speaker) the more likely they were to use “the name”.
But the “the” itself was and still is an ingrained localism.
I grew up in Los Angeles very close to the Santa Monica freeway and I concur.
I hadn’t noticed that “the” was mostly only used for the 400 series highways. In practice, I probably use it for others when I lived in southern Ontario because it’s what I did growing up and still do now back in Québec for all highways.
French, of course, would always use “la 15, la 55” so there may be some linguistic influence from that? I really don’t know but I know my two languages influence each other. I recently caught myself calling out to my son to “close the lights!” which made me chuckle when I realized it.
“Close the lights” strikes me as one of those unique Montreal-isms that undoubtedly stems from the interplay of French and English.
Heh, I know close the lights/open the lights from a Filipino ex-girlfriend.
And since a closed circuit would have the lights on and an open one would have them off, it really kind of messed with me when she said it.
Don’t think circuits; think a shuttered lantern.
Oh yeah, I understood where it came from. But my brain didn’t go to the lantern idea immediately, and I’d sit staring dumbly for half a second before it would click.
TTBOMK (to the best of my knowledge), in the United States, “the” before a route number is rarely used outside of southern California.
Isn’t that also true about all the parkways and expressways in the greater NYC area?