btwhite92 (2019-07-20 16:41:54+00:00): Since these changesets undid a lot of the road class changes I made here, I should probably chime in. Lots of words and opinions ahead:

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I agree with mapman44 that the ODOT classifications for both highway class and expressway are generally correct by OSM standards (can't say the same for many other states!).

With the OHP Highway Class layer on: 

- Red is interstate which should obviously be 'motorway'.

- Green is a "statewide" route which nearly always correspond to 'primary' roads.

- Blue is a "regional" route - these ones are trickier. If the route is the best way between two decently sized cities where no other interstate or freeway services, then it should be primary. If the route only connects small towns or is mostly paralleled by a freeway, it should be secondary. Example - OR 99W is correctly classified as a primary for being the best way to get from Eugene to Corvallis. OR99E is INcorrectly classified as a 'primary' since it connects small towns and it mostly parallels I-5 through the Willamette - it doesn't really provide routing that the interstate doesn't do better, so it should be 'secondary'. They are both "regional" routes by OHP standards.

- Brown is a "district" route - these are nearly by definition 'secondary' roads ("A highway which is not part of a major route, but nevertheless forming a link in the national route network."). OR 238, a "district" route, is now incorrectly classified as a 'primary' when it should be 'secondary' - its function is mostly to connect Jacksonville, a mid-sized town, to more important roads. It is not an important, cross-country route, nor a major city artery, but instead a link in the network of more major roads. Ashland to Klamath Falls is right on the knife-edge of the primary/secondary divide, but ultimately I decided on secondary because OR 140 is the more important route, and provides a roughly similar network function (Rogue Valley <-> Klamath).

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If 'primary' is going to be used in the US to mean "most important roads in network", then there is overuse of 'primary' in the US. These should be routes that either facilitate major cross-country travel where interstates don't, or are a small handful of the most significant arteries in a major city, whose alignment accomodates significant circulation where no freeway or expressway reaches. Notably, this is why I downgraded OR 99 from Ashland through northern Medford to 'secondary' - the 'primary' route through here is Interstate 5, and theortically OR 99 could be cut off between towns (Phoenix, Talent, etc) without causing TOO much of an issue, since again I-5 is the main artery through here. 

I think a major factor in contributors being eager to use 'primary' for any decently major road is one, that it renders like it 'should', and two, to distinguish them from overclassified minor roads:

- (Opinions ahead!) On point one, I don't think the main OSM slippy map properly represents US roads when the mostly-consensual US classification scheme is correctly used. A better US renderer would render state and US routes at lower zooms regardless of OSM class - this is a discussion for a different time, but the point is, don't choose a higher class for a road simply so it shows up 'better' on the slippy map.

- Point two, many areas have waaaay too many minor routes classified too high. I think this stems from TIGER import classifing a ton of driveways and private services roads as residental, and then going from there. In rural areas, driveways (included shared) should be 'service', minor roads that provide local (often dead-end) access to residences are 'residential', minor roads/highways that collect rural traffic (most rural roads) are 'unclassified', and county-level highways that connect small towns or collect traffic towards region-level routes ('secondary's) should be 'tertiary'. Before jumping to 'primary', it's best to make sure adjacent roads aren't overclassified in the first place. Too often every two-lane road in a rural area with a center stripe is tagged 'tertiary', when many/most of them should be 'unclassified'.

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Moving on to 'trunk', the consensus in the US is that 'trunk' should be used for expressways. These are usually high-speed, divided, limited access highways. In the mountainous west, it is not always physically possible to meet these requirements, so usually the way you distinguish an expressway is by access control - limited, if any, access to adjacent properties, intersections with a limited number of roads, occasional grade-seperated interchanges. The OHP Expressway layer in TransGIS nails this pretty well, and nearly every length road it lists as an expressway should be 'trunk' - even the two-lane roads, so long as you have verified that they have high access control.

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Finally, I would caution against relying much on traffic counts to make classification decisions. Most major rural highways have much smaller traffic counts than any arbitrary urban artery, but the former is usually 'primary' while the latter is usually 'secondary'. Traffic counts are a consideration, but shouldn't be TOO much of a deciding factor.