Apparently, it was designated by a law: https://en.wikipedia.org/wiki/Interstate_5_in_Oregon#Route_description

I feel like I would be repeating myself for the 100th time by saying something like, "name = Highway 79 is redundant because there's already ref= SD 79".

Well, I did try to add shoulder=* to some of the ways. Maybe I'll do it once I have more time.

Why did you upgrade this portion of Nimitz Boulevard to trunk? It doesn't connect to any other trunk roads south of I-8.

Okay... Well, how do I tag these, then?

Oh, okay then. (The "splitting" of CA 56 confused me; this seemed to clear things up for me.) I went ahead and added Ted Williams Fwy back to the CA 56 relations and removed Carmel Valley Rd from them.

None of the ways were ever tagged with alt_name=Heartland_Expressway or anything like that. Even if there was a relation tagged with name=Heartland_Expressway, that wouldn't help since addr:*=* keys and Nominatum (or any data consumer, for that matter) would associate a relation's name=* tag with its associated ways.

Wikipedia says that a certain NHS route is/will be designated the Heartland Expressway. "Highway 79" is already covered by ref= SD 79.

Yes, I'd like that, actually. Speaking of, I actually did join Slack a few weeks ago and have messaged a bit there.

Oh, okay. Would destination:ref= I 15 be appropriate for this situation?

Okay, I'll go change it. Interestingly, it was already primary, but someone else downgraded it to secondary to "match how locals use the roads."

"Unofficial" road designation? That's very interesting because street-level images show what looks like an I-15 Ring Road (that's what the sign says); the now-deleted relation was my attempt to trace out its route by following the arrows, as indicated on the signs. What's up with this? And what's the proper tagging, if any?

Trunk roads do have to connect to other trunk roads and motorways; otherwise, you'll be left with a stub that doesn't connect to any other trunk roads or motorways.

*and to avoid making the route continuity look so weird*

Alright, I guess that makes sense. I'll redowngrade these roads then (they aren't even officially part of US 20 anyway...).

Ah. But that would leave two primary stubs at either end of this roadway, with this one (locally referred to as US 20) being the fastest route between the actual termini of US 20.

The traffic signal signs sometimes display them (some as W, some as N), and sometimes not. Even when they don't for Grand Avenue, they will display them for the intersecting street(s). Furthermore, the signblades commonly associated with residential areas never display them for Grand Avenue, despite always showing them for intersecting street(s).

Source: aerial imagery (Bing)

Okay, I modified the intersection based on my best interpretation of your response. How does it look now?

Yes, kind of. After a user downgraded trunk 𝘀𝘁𝘂𝗯𝘀 to primary, I decided to sort of recreate the trunk roads while forming a coherent network. That same user then told me that these roads are actually (relatively local) arterials rather than being important cross-country/state highways.