I’m glad you asked the question - I was actually meaning to ask about a similar situation. In Montreal, there is a section of Highway 20 between downtown and the airport where the Westbound and Eastbound lanes cross over and are on the opposite side for a few kilometers. I was curious what other example might be out there, as this is the only place I’ve noticed it happening.
Apparently the Coloweirdos have not discovered the exit off of 80 in eastern Nebraska marked for Ashland and identified by google Maps as being in the area of Greenwood. Once they do, that sign will be in serious danger.
Is it mountainous there? As discussed above, the I-5 cross-over around Castaic is mountainous, and the crossed-over topography is supposedly better for big trucks doing the uphill.
the mountains start there, yes.
That’s a relatively new thing. When I first moved here in 1997, the exits were sequential. I used to drive to Bangor a lot and exited at exit 48. Now exit 48 is 135 miles farther south, in the Westbrook area. To this day every time I see the signs for exit 48, I think of Bangor.
Took me years to get used to it. I-496 used to follow the turnpike through the L-A area and what is I-295 used to be numbered I-95.
I believe we regard the highway numbers as something akin to names. The actual name of Blood Alley is 18, occasionally getting a first name like “SR” or “highway”, but calling it “the” 18 would be rather like saying “the Bertha”. Fortunately, it does help us identify those Californians.
I believe the use of ‘the’ in SoCal comes from the freeway names mentioned by Senegoid. You’d take the San Diego Freeway (which doesn’t actually end in San Diego) or the Pasadena Freeway or whichever. So when people started using the numbers, they kept the ‘the’. ‘Which freeway?’ ‘The Five Freeway.’ ==> ‘The Five.’
Nitpick: the San Diego Freeway does indeed reach San Diego, as the I-5 south from Irvine is named such. However, So-Cal’ers rarely call freeways by their official name anymore; it’s much more common to say “take the 101 to downtown” or “take the 405 to LAX” nowadays. I think part of the confusion was that freeway names don’t always follow the same route number – the Ventura Freeway transitions from U.S. 101 to SR 134, the Hollwood Freeway starts as SR 170 and becomes U.S. 101, etc.
I think “the Five” is a socal thing. I wasn’t a norcal convention when I grew up on the I-5.
Here in the Pacific Northwest, I have never cared nor consciously heard anything other than I-5. Maybe you’re hallucinating Johnny on those commutes from Bellingham to Seattle 
I was referring to the 405, which stops at the El Toro Y…
“The (highway)” does kind of make sense as a grammatical requirement of the Socalish language when you consider that about 75% of SoCal real estate is paved roads and highways.