willkmis (2022-02-01 18:34:04+00:00): I had refrained from commenting much on this, as I feel like the acrimony was about more than a road classification. But since the merits of the whole enterprise have now come up, I do want to describe why I think this trunk classification project is useful and an improvement of OSM data.

I want to start by saying to Adamant that I do think local perspective is valuable and important, and I respect your knowledge of the area. For instance, after reading your descriptions of local circulation patterns (and judging for myself), I agree that Market St/CA 273 is best classified as primary, not trunk, and I don't think many disagree. But I also think it's important to come up with a consistent, nationwide way of tagging roads, and to do this we need to incorporate the perspectives of those outside our local regions. Otherwise, our data is useless to consumers, be they routing engines or just people looking at the map. So I don't think anyone has "authority" over the map, and of course no one ever can: the map is editable by anyone. All  we can do is work to establish uniform practices across the country/world and build consensus for them.

As to the merits of this approach specifically, I think there's some validity to the assertion that there exists some vagueness in the proposal's wording, making local verification difficult. I think a lot of people participating in the project (certainly me) would prefer a single, authoritative source to base our road class tagging on. Unfortunately, no single criterion exists in the US which matches OSM's road class definitions, which are based on official UK classifications. A number have been tried (traffic counts, number of lanes, NHS classification, Caltrans functional classes, membership in Caltrans' "Strategic Interregional Corridors" to name a few), but none were found to perfectly reflect OSM's definitions (which are a little vague themselves!). One particular difficulty, as Adamant has alluded to, is that roads in different regions are built differently. So road classes have always had a subjective component, which of course leads to disagreements.

In the UK, trunk roads are classed by their importance to the regional road network, i.e. how they link cities of regional importance. So the approach here, matching the approach in dozens of other states, is to decide which cities are of "regional importance", and decide what the best routes between them are. Certainly, there is room for disagreement here, and indeed there has been. But one principle that has achieved broad consensus across the country is that roads do not lose their regional importance when they enter urban areas, that the best regional route is still the best, even if it's heavily trafficked by locals as well. Another related idea is that network connectivity is important, that there should be no dead ends in the trunk network except in rare cases of terminal geography. So this is why even if a road may have similar characteristics to other primary routes on the ground, its regional importance makes trunk a classification that contains more information for map users in my opinion. This also matches how many other US maps, paper and online, map regional routes through cities.