Hi,
Happy new year to you as well! Thanks for the additional information, and for including the citations on your changesets that I've seen in the last few days. I am familiar with the Caltrans map you linked to, having referenced it extensively when drafting the CA trunk classification guidelines (https://wiki.openstreetmap.org/wiki/California/2022_Highway_Classification_Guidelines).

Unfortunately, I strongly disagree with your approach of standardizing OSM tags to Caltrans functional classifications, because I think doing so makes OSM a less useful and consistent map. The problem is that OSM's highway=* tag definitions and goals do not align well with state-level functional classifications, in my opinion and in that of the cohort of mappers across CA and the US that put together the https://wiki.openstreetmap.org/wiki/United_States/2021_Highway_Classification_Guidance,  (see the exact line there: "However, it is not a good idea to try to establish a 1:1 correspondence between OSM classifications and the state functional classifications. This will result in over-classification, with a high number of primary and secondary roads, and there will often be primary and secondary stubs and dead-ends. States generally do not put much thought or effort into their functional classification maps, especially in smaller towns, where the classifications are often arbitrary and do not always reflect reality.") I agree with that assessment within LA: even in the changes you've already implemented, there are a number of strange primary stubs with no corresponding change in road attributes, such as Figueroa, Grand, and Topanga Canyon, that in my view clearly show the dataset is at best an additional tool in helping to determine classification, not the be-all end-all.

Moreover, I don't think that Caltrans FCs correspond very naturally in a one-to-one way with OSM's highway=* values. "Other Principal Arterial", which you seem to be conflating with highway=primary, in particular appears to me to contain too wide a variety of roads. At the very least, such a correspondence needs to be discussed (either on the community forum or the OSMUS Slack, which hosts a very active discussion on highway classification that I'd be happy to add you to) and documented on the wiki before being implemented. I'm happy to dig into the cases of individual roads and consider their specific context, but that's my view of the general approach here.

Perhaps more philosophically, I disagree with your claim that "The idea is to have uniformity across all mappers. There is no better way to achieve that – instead of having mappers make their own judgment." OSM's goal is to utilize local knowledge to the utmost to map the world, and I don't think we need to defer to a statewide agency that doesn't really care whether Hollywood is an "Other Principal Arterial" or "Minor Arterial", whereas I, someone who lives here, can tell you that through traffic going from, e.g. the Chinese Theatre to Silver Lake would avoid it and take Sunset or SM Blvd instead due to the road's characteristics (high pedestrian volume, frequent signals, narrowness), and that OSM should reflect that.

Lastly, I'm open to discussing the trunk classifications of PCH, though the talk page of https://wiki.openstreetmap.org/wiki/California/2022_Highway_Classification_Guidelines might be better suited for it than here. But I do want to point out that trunk is most certainly not only "for a true expressway and behaves similarly to a freeway" in the current consensus of US mappers, as described in https://wiki.openstreetmap.org/wiki/United_States/2021_Highway_Classification_Guidance. It can be used for that in "areas of high population density" (though note there is a different tag, expressway=yes, that can better portray the same concept), but in general highway=trunk can also be used for "the most important non-motorway roads that provide principal, long-haul connections between population centers of regional importance". I tagged PCH as trunk through Malibu because in my opinion it fit that definition as being the best way to get from Santa Monica to Oxnard, and the southern portion because it's the best way between Long Beach, Huntington Beach, and Newport Beach. I don't see houses along the road as contradicting that definition, just as they don't for US 101 through San Francisco, for US 395 through Bishop, or countless other examples throughout the country. Other road attributes, like lane counts, speed limits, and "expressway-ness", are already covered by other OSM tags, whereas highway=* is the only one that expresses the road's important in the broader network.

Looking forward to discussing this further!

All the best,
Will