[Talk-us] US Trunk road tagging (Zeke Farwell)

[stevea]

On May 7, 2021 at 7:21:52 AM PDT, Zeke Farwell <ezekielf at gmail.com> wrote:
> unclassified/residential --> level 6

While "Levels" 1 through 5 (as Zeke fully describes) largely coincide with my hierarchical sense of road classifications in the US (and California especially), AS THEY ARE LOGICALLY MAPPED ONTO OSM (I must both say and emphasize that in this context), "Level" 6 conflating unclassified and residential is tricky.  Yes, both are "below" Level 5, but residential definitely has the additional semantic of "abutted by or primarily serving residences" (or a small number of them, like two or three, but not a single residence, as I've occasionally noticed — serving a single residence is more like highway=service + service=driveway).  What OSM calls "unclassified," while also below Level 5, has no such sense and can frequently be difficult for "an outsider" to figure out exactly what purpose such an unclassified road actually serves.  Sometimes, "unclassified" is a sort of a "cut-off" road, or in common parlance, "a shortcut."  It may be a "lesser" road, but it is often shorter in distance than more-major roads.  In any hierarchy that conflates Level 6 with both residential and unclassified, we should make it a point to emphasize this additional semantic distinction on residential.

Such "unclassified" roads (I know, "unclassified" in the sense of UK road classifications) certainly do belong at (or near) the bottom of such a hierarchy, although one may or may not be paved (it usually is paved).  When unpaved is where distinctions between unclassified and track get tricky.  If it is agricultural- or forestry-purposed, I'd lean towards track.  If it isn't, and "serves" something else (which can be elusive), unclassified is likely more correct.  And we could start a whole new debate if track fits into Zeke's hierarchy at Level 7, and perhaps bridleways and cycleways at Level 8, and perhaps paths (footpaths) at Level 9.  Because of "mixed-use paths," here be dragons (w.r.t. hierarchy as described).

I agree with Minh that Santa Clara County's Expressways are a bit of an odd duck:  our wiki[1] describes their unique history.  Currently, these are tagged with highway=trunk, as what they are (unfinished freeways / highway=motorway, "Level 1") makes them essentially "Level 2."  I also like "doubling the numbers" as he suggests, so that something somewhat exceptional can creep in-between (similar to admin_level being well-established and very widely agreed-upon with even numbers, while odd numbers are for the "more exceptional, but still real in the world" levels — at least in the USA).

Such "shoehorning" will always, ALWAYS have exceptions and odd ducks that defy widely-agreeable classification.  Even after 16 years of OSM giving rise to Zeke's spitballed hierarchy (with which I roughly agree), and "number doubling" as described by Minh, we could easily need to revisit this in five, ten or another 15 years.

SteveA

[1] https://wiki.osm.org/wiki/Santa_Clara_County,_California#Expressways