It gets even more complicated. If you're from SoCal, it's just a number, e.g., "the 5":
https://www.kcet.org/shows/lost-la/the-5-the-101-the-405-why...Canadians use the definite article, AFAIK. Cf
https://news.ycombinator.com/item?id=15229232In any case, I'm not sure there is a clear-cut answer to this question. After all, these roads are not in an English-speaking country.
Since I was curious, I looked up how American news organizations refer to German autobahns (e.g., A67),
Here, it's "highway A3":
https://www.nbcnews.com/http://www.nbcnews.com/slideshow/sno...Now, "the A5 Autobahn":
https://www.foxnews.com/world/several-killed-after-truck-rea...Or "the A14 autobahn":
https://www.merriam-webster.com/dictionary/autobahnMost of the time, I use the truncated version, e.g., the A67 (no "autobahn"), despite the usage of "Take I-67" and "Take 635 to..." and so on.
Maybe it'd be better to spell out the name of the road (if it has one)?
Or, if you say "the SP[number] [type of] road," there is no longer a UK/US difference.
Best wishes