The documentation for turn:lanes says the tag should be applied "from the first indication ... to the junction". What if I know the lane turning indications but not the position along the road where they start (for example, the position where the road widens with the addition of the turn lanes)? Is there a way I can usefully contribute this information to the map? Often, during a trip I'm taking for another reason, I can remember the turning indications but I don't want to go to the extra trouble to try to record the starting position.

So far, I've been adding the turn:lanes tag to the last way before the junction, which may be short or long depending on how the road is currently segmented on OSM (which would depend on what segments needed to be tagged differently for other reasons). In effect, I was thinking of the turn:lanes tag as applying to the end of the way. I believed that even if my practice didn't follow the standard, it was still better than not having the data. And indeed, the main router that I use (OsmAnd) showed the same lane guidance regardless of the length of the last way, as long as it didn't extend beyond the previous junction. But I recently realized that two problems can occur if the way extends beyond the previous junction, which is the case with one way I recently changed (at version 5):

  1. If a route includes a turn at the previous junction, OsmAnd shows lane guidance based on my turn:lanes value there even though it was not intended to apply to the previous junction.
  2. If another user later splits the way, the editor may copy the tag to both new ways, and the fact that I intended it to apply only to the second way would be forgotten.

So now I'm wondering both how I should fix my previous edits and what I should do in the future if I know the turn indications but not where they start. In some cases, I may be able to get the starting point from imagery, but not in all cases. Would the following be reasonable: add the tag to the last way, but if that way extends beyond the previous junction, split it there before adding the tag to the second part? Then is there a way I can record for other mappers (e.g., in a note tag) the fact that I didn't know the real starting point, so that if they do know it, they know it's safe to overwrite my data? (If it would be better to take this discussion to another place, such as the "tagging" mailing list, I'm open to doing so.)

For that matter, is turn:lanes data on OpenStreetMap valuable enough that I should even bother to collect it and address this question in the first place?

asked 08 May, 20:29

Matt%20McCutchen's gravatar image

Matt McCutchen
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accept rate: 0%

Out of curiosity: How are you able to identify the turn lanes without knowing where they start?

(08 May, 22:50) InsertUser

As I'm moving, I can see where the lanes start, but I have no way to locate the point precisely on the map. Even if my phone is recording a GPS trace, if I'm driving, I may not have a free hand to push a button to make a mark at the current location, or even if I did, the latency of my GPS receiver may make it hard to get an accurate location.

(08 May, 23:14) Matt McCutchen

As you have discovered, turn:lanes tagging applies to the entire length of a way. There may be some applications (e.g. lane guidance in some routing software) where this distinction doesn't have a visible effect, but it does matter for other use cases. For example, a renderer with support for lanes will display the turn lanes along the entire length of the way.

Of course, it would be ideal if you could make out the start of the turn lanes on the aerial imagery background or with street-level images that are permitted for use in OSM mapping (e.g. Mapillary, KartaView) and split the way in that location.

If that's not possible, though, you could split the way quite close to the junction and tag only the short bit immediately adjacent to the junction with turn:lane information. This reflects the knowledge you actually have: Only the last bit before the junction will have lane information, because that's what you remember. Lane information for the rest of the street will remain untagged and thus unknown. If a future mapper figures out the exact location where the turn lanes start, they can move the split node (or merge the short way back with the longer section as needed).

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answered 08 May, 23:39

Tordanik's gravatar image

Tordanik
11.5k1498143
accept rate: 34%

edited 08 May, 23:39

1

Thanks. As a newcomer, I assumed that artificially splitting ways was undesirable, but if you don't think it's a big deal, I'll go with that solution for now. Maybe I'll invent a tag, fixme:turn:lanes:start=resurvey, to indicate that the starting point of the turn:lanes is unknown.

(09 May, 01:31) Matt McCutchen
1

I think a textual fixme tag would be better, like fixme="start of turn lane imprecise". If you invent a new tag it won't be displayed by any tool.

(09 May, 18:41) H_mlet

Adding nodes to change geometry is normal. Splitting the way at a node is necessary when way information is different for each side of the way. If you are mapping lane information, you will be doing this regularly unless working in an area where it has already been done. If you have data that likely needs correction such as guessing at the location the turn lane starts then you could use a fixme or a map note to draw attention/reminder to it. If you plan to fix in short order they make it easy to find and you can even note there that you will resurvey it but if not then other mappers can take what is there to further revise it. New tags are best when trying to present something that doesn't fit current tagging. New tags are worth documenting and discussing so people can be aware of them and may have feedback to further improve the idea. I don't know if anyone looks out for fixme: tags but editors definitely draw attention to fixme itself. You could store the entire information within the fixme message if you felt that what you were about to add could be wrong data; someone could copy it into proper tags when ready.

(20 May, 20:28) mirror176
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