Southern Californians (I'm proud to be an Angelino), I thought uniquely refer to highways by using "the" and the highway number (this has even been satirized on SNL) such as "the 101" Â I'm reading a mystery book by Brit Peter Lovesey that refers to heading out on "the A6." Â Do I need to desensitize my phrase radar (Phradar)?Â
I don't find that at all unusual. Here in Arizona, we also refer to highways like that. But then, a lot of what we "zonies" do is stolen from CA. I don't think it's a British vs. American English thing. Now back in the Midwest (where I grew up) we never referred to highways like that. It was always "highway #" or "interstate #".
First time I ever heard that usage was by Johnny Carson doing his classic "Art Fern" shtick, and referring to "THE Slauson Cutoff". For those too young to remember, here's his routine:
There are many highways that have names as well as numbe to 6rs, and they tend to be better known by the name. to the 6 exit
For instance, there's the Cross County Highway north of Cicunnatu. It must have a number, but I don't know what it is any more. US 30 and US 40 are Lincoln Highwat and the National Road, except that when the highway was relocated to upgrade the road, the numbered road moves, but the named road remains the same.
I'm not sure where Dixie Highway starts or ends. When I was going to the University of Dayton, I thought it was local, but eventually found in other locations as well.
The Indiana Toll Road, and the Pennsylvania Turnpike connect to the ends of the Ohio Tollroad, abd it's either I-80 or I-90 or both, but if you're taking it, it's generally just called "the turnpike" in those three states as if it was one road. In Illinois, it becomes the Dan Ryan.
In many cities, there are shoty highways called "the bypass" and of late, it's an interstate with a three-digit moniker, but often a "bypass" is called "the loop". In Indianapolis, they have an inner and an outer loop, but in Chocago, the loop is a neighborhood rather than a highway.
And while news stories refer to Interstate 69 or Highway 6 or Route 127 or SR 637 or Indiana 101, advertisement and in oral language, people are told to take 69 to 6 and head east to 101, leaving off the full name. I suspect all this is pretty common everywhere. Â
And, in the Washington, DC area, there is the Beltway.
Perhaps you use  The  more where it concerns general discussions,  and drop it where giving direction:
The I-30 goes all the way to Little Rock.
Take I-30 and go east for 10 miles.
To me the rules concerning the article  The  are always mystifying-  you generally feel pretty confident with  the  when  of it,  but the why  is often hard to  explain.