Trunk in a funk

Posted by mvexel on 6 October 2017 in English (English)

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.
  • 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:

  1. The road is designed for speeds > 50 MPH and has limited at-grade intersections.
  2. 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?

Comment from dbaron on 6 October 2017 at 19:40

I wonder if the 50mph threshold ought to allow for some local variation. For example, the Santa Clara County (California) expressway system is tagged as highway=trunk, and I think this tagging makes sense. (These are Central Expressway, Foothill Expressway, Lawrence Expressway, Montagu Expressway, San Tomas Expressway, Southwest Expressway, East/West Capitol Expressway, Almaden Expressway, and maybe one or two others.) These expressways have limited at-grade intersections (and the ones that exist are a mix of ramps and lights); they're clearly more important than a number of the highway=primary surrounding them, but also less important than the actual freeways (which are highway=motorway). Yet in many cases the speed limits are 45mph, which doesn't quite fit your rules, but I think the trunk tagging makes sense.

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Comment from mvexel on 6 October 2017 at 19:53

Yea, the OSM wiki actually has 40 MPH which perhaps makes more sense. Living in a rural state where speed limits are up to 80 MPH colors my thinking I guess.

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Comment from ke9tv on 6 October 2017 at 20:32

Question about 'parkways' in New York. Many of these (e.g. Grand Central Parkway, Southern State Parkway, Northern State Parkway, Meadowbrook Parkway, Wantagh Parkway, Sagtikos Parkway) are full-up motorways (dual carriageway, access fully controlled) - except that they have low clearances (so HGV can't make it through a lot of the underpasses) and are signed 'no commercial traffic' or 'cars only'.

Still motorways? Or should they downgrade to trunks? (I'm thinking 'motorways' myself.)

Note that this is NOT all state parkways. Examples: Taconic Parkway is a trunk for most of its length (there's a section north of Hawthorne to where the grade crossings start that I'm ok with 'motorway'); Long Mountain Parkway is a primary, and Seven Lakes Parkway is (or should be) unclassified.

Also, do you agree with me that the current tagging in is simply nonsensical, whatever we decide? Either it's a motorway with occasional at-grade crossings (I don't like but am willing to defer to the community) or it's a trunk. It isn't a motorway with 30 m of trunk spliced into it for a grade crossing - that's a statement, "Just kidding, it wasn't really a motorway after all!"

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Comment from Your Village Maps on 6 October 2017 at 21:09

I feel I should comment on a disappointment I have with Openstreetmap in general. I think this (and other) "debates" on tagging issues are exhausting and seldom result in definitive decisions that the community abides by. I would like to recommend voting on such things so as to set concrete, unambiguous rules that mappers be required to adhere to.

This issue of trunk vs motorway or trunk vs primary, etc., is just one example. I think that most of us have also have done things such as "Adopt a Highway" where we "watch over" certain areas and certain highways. For example, I have "adopted" US 52 between I-90 and I-94 in Minnesota as one, and this "trunk" discussion is very relevant to it. Recently I "had to" edit many segments of trunk, trunk_link, motorway and motorway_link after another mapper (who shall remain anonymous) had done some very odd edits based on their own understanding of trunk, trunk_link, motorway and motorway_link, which resulted in individual interchanges being composed of a mix of all four road types (motorway, motorway_link, trunk, and trunk_link), which, obviously, looks pretty insane when rendered.

In general, I would like to see all this ambiguities addressed and resolved and communicated well to the OSM mapper community, in the US, and internationally. And yes, I think that differences in some definitions, mainly regarding keys/tags, should exist between different countries, and the community should be educated as to what the differences are so that they do not try to apply rules from their country to another they might be working on.

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Comment from Paul Johnson on 7 October 2017 at 04:25

US 6 between spanish fork and I 70 being trunk is leftover NE2 vandalism, from when he bulk-retagged all US routes that weren't motorways as trunk regardless; and the history on a random sample of segments confirms this. I'd generally consider that a primary; the tiger import got it right on this.

UT 154... I'm not seeing a reason any part of that should be motorway. Seems pretty clearly a trunk through and through. It might later become a motorway if the at-grade intersections intermediate and at the south end is eliminated.

US 89, agreed, that's definitely a trunk between Farmington and I 84. North of I 84, looks like it turns primary and is yet another example of NE2 smash-tagging everything with US in the ref as trunk.

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Comment from theangrytomato on 7 October 2017 at 04:29

Functionally, putting aside being pedantic about tagging-for-the-renderer, the purpose of the "highway=" key is indicate how dominantly to show a given road on a map. It is a measure of the importance of a road in the network - an idea put to, and approved by, a vote (albiet an old one). Physical characteristic, if that is what one wants to render, can be described effectively by other tags (motorroad=, lanes=, oneway=). Especially in the U.S., the necessity of an "importance" tag makes sense, because physical road characteristics do not necessarily indicate any kind of network importance. There are countless high-speed, limited-access, divided, multi-lane, non-motorway roads that service small, far-flung suburbs. There are also many examples of plain old two-lane roads that are as crucial to the transportation network as our interstates.

The "expressway" definition works in many European countries for the same reason the U.S. has very little debate about the "motorway" definition. Expressways are signposted and clear-cut, in the same way that there is zero confusion about whether or not you are on a freeway in the U.S. (barring fringe cases like super-twos & rural western Texas at-grades). Here's a list of expressway signage used in different European countries. This can also be seen on our international highway equivalence guide here. Roads signposted with expressway signage in these countries are tagged trunk. Makes sense! The U.S., outside of cases like the Santa Clara expressway system mentioned above (which are obvious candidates for trunk status), does not have such clear cut definitions. There exists MUTCD standards for "expressway" signage (similar to our "begin/end freeway" signage), but it is rarely used outside of cut-and-dry expressway systems, again like the Santa Clara system. Even on OSM, if someone proposes a definition with a criteria more objective than "it's kinda like a freeway but not really", you can almost always find a road that meets the criteria proposed, but is not so important that it needs to be shown at low zoom.

Instead of thinking of trunk as a demotion from a motorway, I think the U.S. would benefit from treating trunks as the most important roads, with motorway as a qualifier for trunk indicating that the road is a freeway. Every U.S. road atlas ever put to paper works like this, for good reason. Take a look at Canada. Maybe their definition is easier for them to work with since they don't have an interstate system like the U.S, but the road map there works beautifully. It is simple to understand how to get around between major cities. This is mostly not the case in the U.S., because we treat primary as most important, and then upgrade sections to trunk when they exhibit freeway-like features. One of the more egregious examples I have seen of this is at the center of the map here. You can almost see that there are two important, cross-country routes through Kansas, Colorado, and Oklahoma. But you can't really, because we've insisted that only the grade-separated portions are trunks. If you zoom in, you find a sea of primary roads that provide little illumination as to which routes you would use for cross-country travel. Examples similar to this are peppered throughout the U.S., which I don't understand at all. What map consumers, aside from highway engineers, are benefited by seeing in very obvious terms, when a two-lane road has a grade-separated interchange or shortly turns divided as it circumscribes a town?

A "most important road in the country that's not a freeway" definition for trunk de facto includes nearly all bone-fide expressway systems, so it doesn't affect how most U.S. mappers are already using this tag. This definition would allow us to do away with having randomly peppered trunk segments scattered among a sea of primary roads, and instead build a more useful and cohesive road map. Maybe "most important non-freeway road" is just as vague as "it's like a freeway but only kind of", but we have been using this definition with the primary to unclassified spectrum for years without too much trouble. The local & consensus-based approach to mapping is excellent for its flexibility (important when trying to topologize the entire Earth!), but I think it falls a bit short when it no consensus can be reached on something as fundamental as the "highway" tag.

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Comment from kucai on 7 October 2017 at 23:48

Aye, same thing happened here in Malaysia. We (our less than a dozen mapper heh) kinda suspended using the trunk tag for the moment.

Some of us wanted to us the technical side of the definition, which eliminates a lot of current trunk tagging and reclassified them to primary.

I argued for using trunk for major intercity connecting roads (most are of the primary type in classification) that were built during the British times before we started building modern tolled motorways. These are usually given a ref number of value less than 10 since they are the earliest built intercity roads.

No consensus, therefore we just freeze the tag usage.

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Comment from Minh Nguyen on 9 October 2017 at 06:05

The wiki’s Ohio page suggests highway=trunk based on some physical characteristics, but I think it would be more convenient to say that each of Ohio’s trunks meets one of the following definitions:

  1. Urban expressways built before construction began on the Interstate system and never upgraded to freeway standards (e.g., due to space and funding constraints). The quintessential example in Cincinnati is Columbia Parkway, a four-lane, undivided, 45-mph expressway.
  2. Rural highways somewhere along the decades-long process of being brought up to freeway standards, starting with limited access along rural segments and freeway bypasses around major towns (which are tagged highway=motorway). Examples in Southern Ohio include US 23, US 35, and SR 32.

Unlike a lot of the trunk definitions I’ve seen so far, these definitions are intentionally flexible about grade separations, speed limits, lane counts, or length. Every urban expressway in #1 was built to a different set of (now obsolete) standards and every rural highway in #2 is in a different stage of development, yet all of them are essentially stand-ins for freeways.

Despite long stretches of simple two-lane roadway, the AA Highway in Kentucky satisfies #2; I think that would better meet the needs of map users in that rural area than any strict definition based on physical characteristics. This makes me think these definitions might be flexible enough for other parts of the country too. However, one challenge is that they require local knowledge – a bit of historical context beyond what you could necessarily glean just by driving down the road.

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Comment from mvexel on 13 October 2017 at 18:16

Hi all, Thanks a lot for your thoughtful responses. Here's a followup I just posted to talk-us:

Hi all,

I haven't abandoned this thread or thinking about it. It has just taken me a while to read through all the diary comments + what is being said in this thread. I intend to follow up with another diary post where I try to collect this smart crowd's thoughts and suggestions, but it will probably not until after State of the Map US that I get to this.

In the mean time, I decided to test some of the ideas posted here on a real case: The part of Michigan SR 10 northwest of the I-696 interchange:

Since 1) this road does not seem to serve an important connecting role in the long distance road network 2) the density of abutters and related driveway / parking exits I judged a downgrade warranted. Please discuss here or on .

On the topic of tagging for the renderer, two things: 1) A US-specific rendering would be really neat 2) Trunk 'appendices' like the one I just downgraded do make rendering at low zooms tricky -- you end up with short segments that seem to end in nothing.

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