User talk:Bicycle tourer/Confluences

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Overall summary

The rules how to classify highway links (primary, secondary etc.) do not lead to useful results if one road merges into another (confluence). The existing comprehensive list for different classifications of involved roads misses a distinction of different types of junctions and therefore produces wrong results for certain types of junctions (like a confluence). Inserting such a distinction (including criteria) into the wiki page for highway links would create a monster page (the confluence case alone leads to reference pages of certain size, which are separated from the confluence page). To solve the problem of bad rules and defining the right rules for confluences, several things have to be done in parallel, which is the content of this proposal, including suggestions for appropriate wiki pages:

  • Introduction of classification of different types of junctions like intersections, confluences, roundabouts etc. (tbd in a new page for junctions, no suggestion yet available)
  • Introduction of confluences (German "Einmündung") for places, where one road merges into another one (Main page to this talk page)
  • Types of confluences (here is the proposed reference page)
  • Rules how to classify links in case of confluences (see this proposed reference table)
  • Rules how to classify links in other cases (as an extension to the existing page about highway links, tbd after acceptance of the above)

Expected result after acceptance

After deploying this, esp. after making the classification rules more specific to the different cases (this proposal focuses on confluences), there is a good chance that a real consensus is achieved about these rules (hopefully visible by vanishment of heated discussions/islands of private rules/resignation of mappers) and people know how to map and classify highway links in different cases (at least they know where to look at).

What get's different? Is it already used?

  • Example 1 would have its links classified as primary if the generic ("wrong") Wiki rules are applied. It is mapped against theses rules (the links are tertiary) but as many confluences and according to the specific rules proposed here.
  • Example 2 would have its links classified as tertiary if the generic ("wrong") rules are applied. It is mapped against theses rules (the links are residential) but as many confluences and according to the specific rules proposed here.

It is difficult to proof how often confluences are already mapped according to the proposed rules, because a confluence is an overall situation with many elements and not a single element where tag values can be counted. But private rules of mappers (seen e.g. in the German user forum) show, that they us rules as proposed.

The proposal and how it came up

Introduction and current status

The idea of confluences came from a german 3-way-junction, where introducing separate carriageways (links) in a highway=secondary (let's call it M for merging) forced an assignment of the new links away from M to the receiving road R (where M is merging into) (e.g. Reason for this were the rules in OSM wiki (Link_roads_between_different_highways_types). Discussion in the German user forum (, sorry, it's German) showed, that everybody was extremely unhappy with this result, including myself, who did the change according to Wiki and brought the issue to the forum. It furthermore turned out, that the majority of people participating in the discussion has given themselves rules how they map such junctions. Their rules were clearly different from OSM wiki, but surprisingly they were pretty much the same for all of them, without even having discussed this.

After reading these comments it was clear that something is broken with the rules how highway links should be classified (it is marked as under discussion in the wiki page). When having a glance at the wiki page about links, esp. the talk to it (without looking into history), the intensive discussion about the rules was obvious. Conclusion: There might be rules written down, but the results they are producing are not overall satisfying (to be polite).

After looking at some more wiki pages I observed, that the whole wiki tries to focus on atomic elements: ways, nodes, attributes to them (tags). It is clearly understood that with the given history of OSM this is a "natural" way. But discussions like classification of highway links show that this might not work good enough in more complex situations, where the environment, the overall situation is the decisive one for the "right" tagging (producing satisfying results). For the junction triggering this page this means: The current generic rules produce wrong results, specific rules for confluences as proposed here lead to results according to expectations.

So I decided not to jump straight into the (heated?) classification discussion for highway links, but to make an effort to define the overall situation of a junction and offer some type of guidance when a situation like a confluence appears, including how to map in these situations. I've seen several wiki pages or forwards for some (other) types of junctions (there is high probability that some are missing in the following list):

  • Junctions: explaining intersections roughly (I wish under this title were an overall introduction into junctions, defining which different types exist, and having explained intersections on a separate page)
  • roundabouts: forward to Junctions
  • roundabout: an explanation text which types of roundabouts exist (not complete, e.g. circular missing)
  • Tag:junction=roundabout: a comprehensive explanation of situations of roundabout and how to tag (it could be discussed whether this content should have another title because it discusses much more than this tag, but this is not the question. More important: It's still a very good page for people who want to learn how to map roundabouts. At a first glance I consider this something similar to what I want to achieve with a wiki page for confluences.

Not intended

Please don't get me wrong: Even if I consider the overall approach of my suggestion different from the structure I see in most of OSM wiki, I'm happy that this wiki exists, and I'm happy that tags are explained. So I'm not proposing a major overhaul of wiki pages. I only want to add the view of a mapper, who wants to map a junction, but gets worried about how to classify links. But he can immediately identify that his junction is a confluence, as soon as such a distinction is offered. And then jump there, get the rules, be satisfied with the results ...


Choosing the term confluence

I was wondering very early which term would be the right one to choose in english language. The German "Einmündung" is understood and used for streets/roads by (almost) every German speaking. It's direct translation is confluence (so told LEO). Unfortunately confluence is used in English language only for rivers (again LEO). So what to do?

The German user forum provided several ideas for English language:

  • T-Junction: I initially liked T-junction and started my very early work with this term, but than I saw a T-junction, where the main street coming from south made a turn to the east, while the merging street coming from west was just continuing without even a slight turn. Because people might expect from the pure appearance of such a junction that the one coming from south is the merging one, (which will lead to wrong application of rules for highway links), I decided that T-junction is not the best.
  • 3-way-junction: Well, this does not underline so much, that one road is merging into the other.
  • Merging ...: I did not find a single noun for the verb "merge". "Merger" I disregarded because everybody will associate this with two companies merging ...
  • branch: Branch points into the opposite direction than merge. To better understand why people are not happy with the current highway link classification rules it helps to look at it from the direction of the merging street to the merging point, not from the branch into the diverging street

So my current choice is confluence, because (for my feeling) it is a noun, and it supports that one road is merging into another. I would be happy for any advise from especially native speakers.

--Bicycle tourer (talk) 00:40, 21 September 2018 (UTC): I learned in the meantime the definition of T-Junction in Wikipedia (Three-way_junction). I'm not sure whether confluence makes any difference but I will not change back to T-Junction until I've got some advice from native speakers.

As soon as a junction is two confluences in parallel (example given in main page) there are several arguments not to use "T-Junctions". So there is a differenc between a T-Junction and a confluence, but again, let's wait for native speakers --Bicycle tourer (talk) 16:00, 23 September 2018 (UTC)

Talking about roads and not so much streets

This is again a language sensitive issue. German language only knows the term "Straße", which sounds pretty similar to English "street". There is no separate German word for road, probably "Landstraße" is nearest to it but has other additional meaning. Nevertheless I expect (without having counted) the majority of confluences to be "out of town", esp. if you look at those, where the classification as highway matters and links exist. That's why I prefer road and talk about roads in this proposal.

Why is this proposal located in user pages

The official way for a feature-proposal would be to generate a new wiki page under "proposed". But several things are different:

  • This is not a proposal for a new feature (tag) in just one single page.
  • The proposal has impact on existing pages, it suggests to change existing pages. So if this impact should be made visible earlier, the existing pages have to be copied somewhere else (my user pages) and adapted here

So to avoid changes in existing Wiki pages I created everything in my user pages as sub pages. Of course if the proposal is agreed and chosen, everything has to be moved into the official parts of OSM wiki.

Why a separate wiki page for confluences

There already exists a wiki page for roundabouts, which is just another type of junction, so the structure of having separate wiki pages for different types of junctions is already there. The wiki page titled Junctions already has a link to roundabouts, I expect to have an additional link for confluences there too. Adding the content of confluence directly to the junctions page will create a monster page.

Open issues

A link or not a link

In case of different carriageways for different destinations in the merging road M it seems to be not clear, whether

  1. both carriageways should be treated as links
  2. one of the two is treated as the major carriageway and the other as link
  3. none is treated as link

There are pro and contra arguments, and it's probably only depending from the personal point of view what to choose --- no easy situation to get an agreed preferred way how to map. But let's look at the arguments.

Both carriageways treated as links

As soon as the carriageway for one direction splits into two separate carriageways for the different destinations both are connections to another road than the merging one. Virtually this can be seen the following way: Let's assume the merging road is not merging, but crossing, because it's continuing on the other side. Let's furthermore assume that for all 3 directions (left, through, right) separate carriageways exist. Then the through-carriageway is treated as the main, while left and right are treated as links. Back to reality: get rid of the not existing continuation and any type of carriageway needed solely for this. Well, the remaining carriageways are both links, and that's the argument for treating both as links.

One is main, one is the link

There is one easy argument for this view: As soon as one carriageway remains being used in both directions, this is treated as the main. Any other carriageway has to be oneway=yes and because of this difference is the link. In case there are 4 carriageways (for each direction * each destination) still the inner ones can be treated as the major

None is a link

Well, because it's a confluence, the merging road is ending. Therefore there is no "through", no need for an exit and no need for a link (esp. in case of motorway/trunk, but also for primary, secondary, tertiary). The carriageways can just be treated as the main roads. And by the way, if you have an unclassified road splitting for the different destinations, there is no choice for a link at all, because it simply does not exist in OSM.

Conclusion for "a link or not a link"

The rules suggested in Chapter "Classification of links" are somewhere in between 1 and 2:

  • As long as the middle road is not split, treat it as the main road and the other as a link (solution 2).
  • If the middle road is split and there exists a third carriageway treat all of them always as a link (solution 1).
  • If the middle road is split but no third carriageway exist treat as separate carriageways for each direction, but still them main ones (solution 2).

This talk chapter has been introduced because of very early feedback. It might be that the proposed rules regarding "a link or not a link" have to be recalled and replaced by something like "any of the above is right". Let's look for further feedback.