Proposal:Turn lanes (way)/FAQ
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In several discussions concerning the proposal, there were many similar questions asked. So I've compiled this page to help you understand proposed tagging scheme.
- There are 20 KB of text! How am I supposed to read all that?
- Actually, most of it are examples. You can skip them and read only descriptive parts: turn lanes, their locations and restriction relation. When you've encountered 'Examples' heading, skip to the next part.
- Why not relations? How do I specify lane properties?
- You don't. This proposal deals with the number and location of turn lanes, but does not state how each lane's properties are tagged. It treats a road as a whole, not as a set of lanes. Obviously, lane properties, as well as permissions to change lanes, can be introduced in another proposal.
- There are some questionable tags you're introducing. Why not something like lanes:forward:4:bicycle?
- I know that this is by far not the first attempt to create lanes tagging scheme. I started it because an other proposal, relation-based, was gaining strengh, and I don't want every bit of highways covered in relations. So I've set three restrictions:
- No relations. Or try to minimize them: do not use relation for things that can be done with tags. Think of Potlatch users.
- No parametric keys. They are very hard to process. For example, there is no way to use them in shape format, and querying a database is unnecessary hard.
- No programming in values. At first I've invented lanes:directions tag, examples of which remain in Building lane directions section. But it doesn't cover every case, hard to read and even harder to understand by a novice who stumbled onto that tag while beginning to map. It is not obvious. Tagging scheme must be simple, and not involve strange letters and punctuation.
- Applying those (with some others, like "do not change the meaning of existing tags"), I've made the proposal you've hopefully read. I doubt there can be another way, considering those restrictions.
- What about bicycle lanes?
- This proposal extends lanes=* tag. It contains a number of lanes for motorized vehicles. The community decided that bicycle and pedestrian lanes are not included in the count (according to Vienna Convertion). Then, bicycle lanes tagging is a separate matter, described in depth on the relevant pages (specifically, cycleway=lane).
- You are changing the meaning of the lanes tag! And what about that new lanes:psv?
- No, I'm not changing anything, just extending the schema adding several new tags. lanes still contains a number of lanes, as well as lanes:*. As for lanes:psv=*, it was introduced a year ago, and was used long before that.
- Too many tags! This proposal is too complex.
- Actually, this is best solved with trying to map a couple of intersections. In 95% of cases you just need to add one tag (or none at all). There are some complex intersections where you'll have to use three tags, or even specify a location of some lane (usually psv one). And I believe that there's next to none cases when one will need to use a restriction relation -- but I provided it just in case.
- Do I really have to map this lane arrangement with four tags (turnleft, turnright, through and through:location)?
- No, single lanes:turnleft would suffice. The default arrangement (without turn lanes tags) is that you can go through using all lanes, turn left from the leftmost lane (if there's a turn allowed) and turn right from the rightmost lane. You use turn lanes tags only if there's a lane you can't use to go forward, or there are several turn lanes for a direction. This is the case: the left lane is only for turning left, and the right one is standard.
- How do I tag center lanes? The proposal does not seem to cover that important part of a road.
- No, it does not. Turn lanes tags deal with the one-way part of the road (and is extendable to either parts with :forward/:backward suffixes), and center lane is a part of the whole road, not any side of it. Actually, there already is a tag for center turn lanes: center_turn_lane=yes.
- Why lanes:turnleft:forward and not lanes:forward:turnleft? The latter notation seems more logical.
- Because :forward is a suffix. It occurs more often at the end of a key than in the middle of it. Most editors (notably JOSM) recognise forward/backward suffixes and switch them when a way is reversed, but do not do so when those are in the middle. And the last argument, this proposal introduces lanes:turn* tags (extendale with standard means), not lanes:forward/backward:turn*: main part of a key is fixed, and it encourages mappers to split road into two ways for complex cases.
- Navigational software needs not only the arrangement of lanes, but also where does each lane lead. How does your proposal supply that?
- It is not the matter for the proposal. But it can be done, and I've built an algorithm during the research for this proposal. In short, for each intersection you already have directions for each lane, source and target way, and a common node between them with all other ways. The task is to determine, which directions (left, right, forward) correspond to which ways - so the target way can be identified as, for example, right turn, and a driver will be lead to lanes that are for turning right. This means, we have to find a way that is a 'through' direction, a continuation of a road.
- If there is a type=[lane_]restriction relation with restriction=only_straight_on tag, you've got an answer. Otherwise, all ways leading from the common node must pass a series of tests. First, is there is lanes:turnleft tag on the source way, drop the leftmost way. The same for lanes:turnright. Then choose way(s) with the highway class higher or equal to the class of the source way. Then the one with the matching name/ref. If there are no names, choose the one with the minimal deflection. In the end, there will be only one.
- Heavy vehicles are prohibited on the third lane — how to tag this?
- In earlier version of the proposal there was a tag lanes:hgv, and the answer would be lanes:hgv=2. But then I decided that it is not a subject for this proposal and can be added later, in a separate proposal. Right now lanes:hgv tag is almost unused, and always means that heavy goods vehicles cannot leave psv/the rightmost lane.
- How should I tag tram tracks inside lanes?
- Right now there isn't a way. I though about it for a couple of days, even started to add it to the proposal, but it's just too complex matter to deal with, and there is no immediate need to complicate the proposal. I agree that tram lanes should be tagged, but there are too many details to account for: sometimes cars can use them, sometimes not (and it usually depends not on physical properties, but on road signs or Highway Code), tram lanes can be mapped with separate ways, one could want to mark a quality of tracks, and so on. We will have to think about all that and extend the lane tagging scheme later, with a different proposal.
If you still have any questions, even tagging ones ("how to tag that crossing?"), I'd be happy to answer them on the proposal discussion page.