Here is how Wikipedia defines “Trunk road”:

A trunk road, trunk highway, or strategic road is a major road, usually connecting two or more cities, ports, airports and other places, which is the recommended route for long-distance and freight traffic. Many trunk roads have segregated lanes in a dual carriageway, or are of motorway standard.

‘usually’.. ‘many’.. Adverbs that serve to muddle the definition: I still don’t know whether a specific road can be classified as a trunk or not.

The OSM wiki has this to say:

Use highway=trunk for high performance or high importance roads that don’t meet the requirement for motorway . In different countries, either performance or importance is used as the defining criterion for trunk – see #International equivalence and Highway:International equivalence for guidance on road classification in different countries.

Hmm. Equally noncommittal. But there are reference to places where more specific references are to be found. I am interested in the United States. So let’s look there. The ‘International Equivalence’ section on the highway=trunk page says:

Surface expressway: A relatively high-speed divided road (at least 40 MPH with a barrier or median separating each direction of traffic), with a limited amount of intersections and driveways; or a major intercity highway. This includes many U.S. Highways (that do not parallel an Interstate) and some state highways. Wikipedia reference

..whereas the separate ‘International Equivalence’ page says for trunks in the United States:

Limited access highway with occasional grade level intersections, or major intercity highway where no motorway exists.

Not precisely the same, but I am starting to see a pattern. The definition of trunk, according to the people who wrote the wiki pages, seems to be a mix of technical and functional road classification:

Technical : Designed for speeds > 40 MPH, limited at-grade intersections.

: Designed for speeds > 40 MPH, limited at-grade intersections. Functional: Major inter-city highway where no motorway exists.

Let’s try and apply this to some major roads in Utah that I know well and are currently at least partly marked as trunk . (This Overpass query shows all trunk ways in Utah.)

US Highway 6 between Spanish Fork and I-70, currently marked as trunk in OSM. This is a two lane road with a speed limit of 65 MPH, with some exceptions in places. It is the main connection between the Wasatch Front and southeast Utah and southwest Colorado. There is no freeway alternative. **Conclusion: proper trunk .

State Route 154 or Bangerter Highway as it is known locally. Currently marked as trunk but some stretches are marked motorway as well. It is a 4-6 lane divided highway. Some sections have at-grade intersections (Continuous Flow Interchanges among them) but they are spaced pretty far apart. SR 154 is not a major inter-city highway, and there is a reasonable freeway alternative available. However, SR 154 does serve an important connector function between cities and towns west of Salt Lake City and the SLC International Airport. Conclusion: trunk is OK, but motorway sections should be downgraded.

US Highway 89. Currently only marked trunk between Farmington and I-84, where it meets the technical (if not the functional) definition of trunk . Most of the rest of US-89 is two-lane road with a speed limit of 65 or 70 MPH, with local exceptions. If you look at it purely from a functional perspective, only the stretch between Brigham City and the WY border in the North, and the (long) stretch from Provo south to the AZ border can be considered trunk . The section actually marked trunk currently is not part of either of these two. To my mind, the sections that serve important long distance connecting functions should be trunk as well. Conclusion: more sections should be trunk .

Looking at a few of the roads I know and their current tagging in the context of the current wiki definitions, my overall conclusion is:

A road should be tagged trunk in the United States if either of the following conditions are met:

The road is designed for speeds > 50 MPH and has limited at-grade intersections. The road is a serves an important inter-city connector function, and there is no freeway alternative.

Small stretches where condition 1) is not met, for example a reduced speed limit in a built up area, should be tagged trunk to maintain a continuous classification.

Pretty? No. Works for me? I think so. What do YOU think?