Just to pile on, this fifth-generation Oregonian lives by I-84, not “the 84,” but the portion between what is now I-205 on the east side of Portland and the western terminus of I-84 at I-5 was known as “the Banfield,” for the Banfield Freeway. (Wikipedia says it was called the Banfield Expressway, but I don’t recall any road in Oregon ever being called an expressway. Actually, the Banfield was more colorfully known in our circles as “Old Blood 'n Gutty” for the number and severity of accidents on it in the early days. Of course, the speed limit on this less-than-perfectly banked section was then 70 mplh, which was routinely exceeded.
But do New Yorkers say it the way we do farther north? Take Interstate 495, for example. It’s not referred to as “the 495,” “the I-495” or “I-495.” It’s simply “495,” as in “When I had to travel back and forth between my home in NH and my grandfather’s in MA during senior year, I took 495 most of the way, then 125 to 101.” 495 is an interstate, and 125 and 101 are not, but how they’re spoken of doesn’t change to reflect that. Oddly, we take “route” 1, 4, and 9, but mostly drop that once you get to double digits (I’ve occasionaly heard “route 16” but not often outside of commercials).
IME, pretty much, but it may vary a little bit depending on context. For example, Interstate 90 could either be “I-90” or “90”, but you might use the “I-90” construction if you wanted to add a slight emphasis to it, such as when giving directions or if you were talking with someone who didn’t drive on it regularly. In more casual conversation or if you were talking with someone who knew it well, you’d just say “90”. And yes, it’s always “route” 1 and 9.
Hell, where I am from (Ky.) we don’t even say the “I.” It is just 64, 75, 71, 265 and 264. Maybe we are just lazier.
Yep, it’s a hybridization.
SoCal: “The 5” (No ‘I’)
Other, lesser places: “I-5” (No ‘The’)
Hybrid: “The I-5” (Doesn’t sound right to anyone)