In the United States for example, if there is a route relation created for Interstate 5, do the individual ways need to also have name = Interstate 5 and ref = I-5? Another example: N 2 in Senegal. What is the current best practice for numbered route naming?
If a user needs to unglue two segments in this relation to correct way alignment, a warning pops up in JOSM that this act may have broken the relation. What effect does a broken relation have?
To experiment (without uploading), I split a way of I-5 twice and deleted a portion of it. The warning pop-up appeared, but there isn’t any change to the relation, as far as I can tell. Perhaps the way in position 86 has been removed, but I see no practical consequences of this removal, like an issue with the name rendering on the remaining members of the relation.
Highway route relations in the US are an attempt to deal with the fact that any one segment of road may be part of different numbered routes. For example along I-5 in the California central valley, there are sections that are also CA-33. One way to possibly deal with that would be to have those sections have a multi-valued ref tag. But there are issues with that too.
If you search the talk-us email list archives from a couple years back you can read some of the discussion. Here are some Wiki pages related to that too:
There is/was a test setup to show how the route relation setup works to help in rendering places where a single piece of pavement is considered to be part of several US and state routes, etc. And from time to time there is discussion on having the US OSM community host its own rendering that is more suited to the US highway system than the worldwide generic map at www.openstreetmap.org.
Thanks for the response, n76. I’m hoping to get further clarification on the practical uses, best practices, and general policy of numbered route relations. I also hope my US-centric example did scare away any other responses! I am looking for more general information, useful when mapping all over the world.
Is it best practice to use relations instead of name or ref tags?
What effect does a broken relation have?
If I could find any answers to these questions, I would be more than willing to update the wiki with more helpful information.
AFAIK, in Western Europe, many highways have the tag “ref”, e.g. ref=E19. In some countries it is common to have a national number as well, e.g. nat_ref=A1 There are relations as well, but they do not have any effect for routing. I think they even have no impact on the rendering in osm-carto (standard view on osm.org).
I would suggest to look at the current tagging of the highways in your neigborhood to understand how local mappers deal with the situation.
The ordering of ways in a relation has no practical use. It doesn’t effect rendering of the numbered route name.
Breaking a relation means there is a gap in the ways that are part of that relation. This does not adversely effect rendering or have any practical negative impact to the map. It’s not ideal to have a missing way, but it also doesn’t harm the data.
Some countries, like Senegal, are using relations and removing ref and name tags for numbered routes, which could be a problem for any maps using OSM data but not rendering the relation data. Other countries aren’t jumping on this bandwagon quite yet, so the use of relations really does depend on where you’re mapping.
I hope this clears up other’s confusion about relations a bit.