# Talk:Key:centre turn lane

(Redirected from Talk:Key:center turn lane)

## Combined key for all kind of reversibles

Please have a look at wikipedia. I would recommend a single key which combines all kind of reversibles, e.g.

``` reversible_lane=[ median | passing | reversible | no]
```

whereas the values mean:

Value Description
median Median/center turn lane: a single lane in the center of the road into which traffic from both directions pulls to make a left/right turn
passing Passing lane: a lane in the center of a highway meant for passing in both directions. Neither direction has the right-of-way, and both directions are permitted to use the lane for passing.
reversible A lane used for traffic in one direction at one time, the opposite direction at another time.
no Just for convenience

Please note, that this key alone doesn't solve the reversible problem. We would need conditional values for this to be solved, but this is another problem and should be solved preferable in a generic way.

I suggested such a common key in my current proposal, but excluded it from the voting, because it still needs some polishing (e.g. I didn't include `reversible` in my suggestion, which might by a mistake).

The following is just a very rough idea how to solve the (bigger) reversible problem. It demonstrates, that it would be more useful to use a common key for all kind of reversibles. It uses the :lanes extension as proposed and it uses some kind of extension for conditional values, which was just discussed here up to now:

``` reversible_lane:lanes=no|reversible|no  1)
reversible_lane:lanes:conditional=no|median|no [time(10:01-15:59)]  2)

direction:lanes=backward|both_ways|forward  3)
direction:lanes:conditional=backward|forward|forward [time(06:00-10:00)]  4)
direction:lanes:conditional=backward|backward|forward [time(16:00-20:00)]  5)
```

Explanation:

1) Three lanes, the lane in the middle is by default a reversible lane

2) From 10:01 to 15:59 the lane in the middle is a median/center turn lane

3) By default the lane in the middle runs both ways (for the center turn lane)

4) From 06:00 to 10:00 the lane in the middle runs forward (as viewed by the osm-way direction)

5) From 16:00 to 20:00 the lane in the middle runs backward (as viewed by the osm-way direction)

Please think especially of the conditional keys as what they are right now: just ideas. Please don't throw anything in my direction for it ;-)

--Imagic 08:47, 8 March 2012 (UTC)

Big problem: sometimes more than one lane is reversible. And a turn lane is really not reversible, as it handles both directions simultaneously. --NE2 19:09, 8 March 2012 (UTC)
According to wikipedia "A reversible lane (British English: tidal flow) is a lane in which traffic may travel in either direction, depending on certain conditions." This is valid also for center turn lanes as well as passing lanes.
Furthermore it states "Most often done on three-lane roads, the reversible lane is typically used for traffic in one direction at morning rush hour, the opposite direction in the afternoon or evening, and as a turning lane at most other times." If you use different tags you will have to use more tags if you want to describe at what time a lane is a center turn lane and at what time it is a reversible:
``` reversible:lanes=no|yes|no
reversible:lanes:conditional=no|no|no [time(10:01-15:59)]
center_turn_lane:lanes:conditional=no|yes|no [time(10:01-15:59)]
```
This is unnecessary complicated.
Additionally there is no problem if there are more than one reversible:
``` reversible_lane:lanes=no|reversible|reversible|no
```
Now there are four lanes and the two lanes in the middle are reversible.
One final note: "reversible_lane" might be the wrong word. In OSM we use british english and therefore "tidal_flow" or something similar would be more appropriate. --Imagic 07:51, 9 March 2012 (UTC)
The Wikipedia definition is a bit vague, but I've never heard one of these called a reversible lane. If you have a proposal for tagging such, please create your own proposal. --NE2 17:42, 9 March 2012 (UTC)
I thought you wanted comments on your proposal. OSM is a worldwide project, not a U.S. project. Therefore any tag should be so general, that it can be used worldwide. The key center_turn_lane is in my opinion too specific. A more general key would cover all situations worldwide and is therefore preferable. --Imagic 18:07, 9 March 2012 (UTC)
I fail to see what cramming everything into one tag has to do with worldwide applicability. center_turn_lane applies to anywhere there's a turn lane, including the U.S., but also countries where they drive on the right: http://www.caymanroads.com/index.php?option=com_deeppockets&task=catShow&id=34&Itemid=31 --NE2 01:46, 10 March 2012 (UTC)
I don't think this is cramming, this is more like consistency. What you suggest would be like using motorway=yes, residential=yes, primary=yes, ... instead of highway=motorway/residential/primary . There are lanes, on which traffic may travel in each direction. Known variants of those lanes are reversible, median and passing. So in my understanding there should be one key. Also because it is not possible, that a lane is a reversible and median at the same time. BTW: "center_turn_lane" is not british english as we use in OSM; it should read "centre_turn_lane". --Imagic 07:34, 10 March 2012 (UTC)
Buried in the text you actually make a good point: it should be centre_turn_lane. (By the way, why do we use oneway=yes rather than restriction=oneway? building=yes rather than man_made=building?) --NE2 21:08, 10 March 2012 (UTC)
What is your problem with oneway=yes? There are at least three possible values for the oneway-key: yes, no and the (extremly ugly) -1. Building? Same here: building=yes, building=shop, building=residential, building=warehouse, ... and so on. And that's exactly my point: all those keys have have many different values.
We should not introduce a key, that simply accepts yes/no as value, if this key describes a property, which may have many different mutually exclusive variants. A centre turn lane can not be a passing lane at the same time, which can not be a reversible at the same time. So why not introduce a key, which accepts all those three values? --Imagic 21:34, 10 March 2012 (UTC)

## Traffic flow / Lane attribute vs. highway attribute

You requested comments. For stimulation of discussion I present two different views on the topic.

I have no clear precendence, but somehow believe that the highway attribute view is easier for mappers. Lane mapping is a rather new development in OSM. Finally the highway attribute approach can co-exist with lane mapping (only one additional tag, which is IMO OK for a few highway that have such special lanes).--Martinq 20:41, 12 March 2012 (UTC)

### Interpretation as highway attribute

`centre_turn_lane=yes` expresses - in a straight forward way - an attribute of the highway: "This highway has a centre turn lane". But:

It is rather uncommon that OSM keys just have yes/no values. Even simply keys like `lit` allow several values/categories. Thus my gut feeling tells me that the key is too specific.

More common in OSM are classification keys. In this case it seems you want to provide more information about the lanes of the highway. Now, the real interesting thing is that there is a lane which cannot be used for 'through' traffic. As already pointed out above, there are similar "special" lanes: Lanes that change direction (tidal flow) and lanes that can used for passing from both sides (suicide lanes) (-> reversible lanes on wikipedia).

Instead of inventing a new key for every type (passing=yes, reversible_lane=yes), we can create one key that expresses that the highway has such a special lane. The lanes have in common that their traffic flow is either time/signal dependent or it can be used from both sides - and they are mutual exclusive because - in a healthy arrangement - they must be in the centre.

```both_way_lane=median / passing / tidal_flow / no
```

'no' can be assumed for a highway without `both_way_lane` tag.

If more than one lane is reversible, I would follow the current idea in the `lanes` tag with forward and backward and specific `lanes:both_ways=2`.--Martinq 20:41, 12 March 2012 (UTC)

### Lane attribute

The both_way_lane is still designed as a highway atrribute. For the `:lanes` extensions discussed in the section above, a lane attribute would be more appropriate. In this case I would specify `traffic_flow` of a lane:

```traffic_flow=median / passing / tidal_flow / forward / backward
```

Directly on a highway the key is clearly confusing and misleading:

```highway=primary, traffic_flow=median   -> Huh?
Compared to:
highway=primary, both_way_lane=median  OK.
highway=primary, centre_turn_lane=yes  OK.
```

Of course it is convenient, to specify a lanes layout like this:

```highway=primary, traffic_flow:lanes=backward|median|forward|forward
```

But I don't see a need for that. Special lanes are really rare (compared to overall number of highways). The lanes must be in the middle, already implying the lane arrangement. Because it is rather special, I would also accept a few tags more in such cases:

```highway=primary, both_way_lane=median, lanes=4, lanes:forward=2
```

- or - with a dedicated lane flow/direction key:

```highway=primary, both_way_lane=median, traffic_flow:lanes=backward|both_ways|forward|forward
```

- or - if you insist on your special tag:

```highway=primary, centre_turn_lane=yes, lanes=4, lanes:forward=2 (or similar)
```

Hope the different view points (lane vs. highway attribute, special key vs. classification key) help you to determine actions for your proposal.--Martinq 20:41, 12 March 2012 (UTC)