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#1 2020-09-23 09:17:33

SixSenses
Member
Registered: 2020-02-12
Posts: 20

Junctions

Hi all,

I find many junctions with 2 lanes streets where a U-Turn in the junction is possible because all directions set to green.

Like this example in Kanthararom.
https://www.openstreetmap.org/edit#map= … /104.58552

Is there a specific rerason for that??

For me, this makes not really sence when navigation tells you to u-turn in the junction.

Greets
Six

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#2 2020-09-23 09:31:38

Russ McD
Member
From: Hereford & Chiang Mai.
Registered: 2011-04-17
Posts: 286

Re: Junctions

I understand your comment - but in the case you show, be careful.  Firstly, if the restriction was added by "Beddhist", then its probably correct.....
I took a quick look, and although I don't know the junction, his tagging makes sense... they are not U-turn restrictions, but no-left turn restrictions ... which makes sense as you have a slip road to use when turning from the secondary to the primary road.
I agree that sometimes the road has one way tagging that can negate the need for a restriction, but this is not often.... I would suggest that you do not change any existing restrictions as they can be complicated, and you can unwittingly undo others hard work.  If in doubt, just sent the contributor a brief message.
Cheers.

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#3 2020-09-23 18:17:21

Bernhard Hiller
Member
Registered: 2011-05-10
Posts: 997

Re: Junctions

I cannot follow.

There is one turn restriction mapped for this junction:
https://www.openstreetmap.org/way/630421172
That is you travel from south east on 2085 and want to turn left into 226. You must not do that directly, but usinmg the link.

I do not see any restrictions on U-turns. Though I'd actually expect to follow 226 to the west to the next u turn place (it was missing, just added it) or to the east (where it is not so clear where it is due to a missing physical barrier).

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#4 2020-09-23 18:57:16

Russ McD
Member
From: Hereford & Chiang Mai.
Registered: 2011-04-17
Posts: 286

Re: Junctions

Benhard ... Now you are really confusing me !  The link u provide is for the Grab discussion ???

Anyway, assuming thats a mistake, I see two restrictions on the junction related to this post .... one on node *9083 and one on node *1683.  Do you count as one for some reason ... maybe because they are mirror images ??

The restriction on doing a U-turn at the lights, are essentially provided by the one-way tagging, which is sort of what I was trying to get over initially.

smile

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#5 2020-09-24 02:48:04

Paul_012
Member
Registered: 2011-08-05
Posts: 199

Re: Junctions

I think the issue SixSenses is trying to raise is that U-turns are not allowed at the intersection, but it's not currently mapped as such. The answer is that there isn't an "all directions set to green" setting. A no-U-turn turn restriction has to be added for every prohibition. It's just that no one has done this yet.

Russ McD wrote:

The restriction on doing a U-turn at the lights, are essentially provided by the one-way tagging, which is sort of what I was trying to get over initially.

I'm quite sure you're mistaken there, Russ. One can always follow the one-way arrows through the loop of an intersection.

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#6 2020-09-24 16:52:44

Bernhard Hiller
Member
Registered: 2011-05-10
Posts: 997

Re: Junctions

Russ McD wrote:

Benhard ... Now you are really confusing me !  The link u provide is for the Grab discussion ???

Oh sorry. Some how the link still stuck in the clip board.
I thought of turn restriction
https://www.openstreetmap.org/relation/11039078
And yes, you are right, I did not see the opposite restriction (ID editor is not my favorite...)
https://www.openstreetmap.org/relation/11039077
from 2086 east into 226.

Still, U turns are possible with the current mapping. Let's take my former example, i.e. we come from south on 2085.
From way
https://www.openstreetmap.org/way/104441405
we continue straight on on the node of the "no left turn" restriction (I do not see any "no straight on restriction" - and that's likely correct, because how else could we get north into 2085?)
https://www.openstreetmap.org/way/104441375
then turn right into way
https://www.openstreetmap.org/way/796710438
then again on the other node of the "no left turn" restriction (I do not see any "no right turn restriction" - and that's likely correct, because how else could we turn from 226 west into 2086 south?) right into
https://www.openstreetmap.org/way/104441453
and eventually straight on
https://www.openstreetmap.org/way/796710435

Violà, U turn achieved.

Last edited by Bernhard Hiller (2020-09-24 16:56:27)

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#7 2020-09-24 20:47:28

Russ McD
Member
From: Hereford & Chiang Mai.
Registered: 2011-04-17
Posts: 286

Re: Junctions

Yeah, Bernhard/Paul ... both of you are correct ... and yes, of course you could make a U-turn ... albeit somewhat dangerous in reality !

One question has always confused me ... technically at these junctions, to make that U-turn, you are in fact doing two right turns in succession.  Logic dictates you need two no-right turn restrictions on two nodes .... so if we use a U-turn restriction, is OSM clever enough to know it just needs to go on the first node.... or do you need to add via nodes ?

Perhaps someone can correctly map that junction with the turn restrictions correctly showing, as even after reading the Wiki, its pretty damn confusing !

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#8 2020-09-25 08:29:21

Paul_012
Member
Registered: 2011-08-05
Posts: 199

Re: Junctions

As an example, I've added a no-U-turn restriction for vehicles coming from the west on Hwy 226. What I usually do is have the three small segments as via ways in the relation. (Having only the small segments as to, via and from should also work, but I think my example is clearer.) Here's how it looks in the iD editor.

l81PIqG.png


There seems to be some controversy over when turn restrictions should be added. The Wiki says to do so only when there are traffic signs explicitly prohibiting a turn, and that routing software should account for default cases where prohibitions are unsigned. In reality, that might not be the case, though. Mapbox has proposed using implicit=yes to tag turn restrictions that aren't signposted.

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#9 2020-09-30 17:48:05

stephankn
Moderator
Registered: 2010-05-04
Posts: 554

Re: Junctions

While driving you have to use common sense. Problem is that often it is not advisable to u-turn. By having this mapped in the data it avoids routing engines suggesting this.

For us we enter here a grey area. If something is "not advisable", how should we verify if a restriction is justified.

In the above example, the situation is clear. On the ground is a turn restriction sign.
https://www.google.com/maps/@15.1001709 … 312!8i6656


Similar potential restrictions are to four-lane highways with a painted island in the middle. Without a physical barrier these are usually mapped as a single highway (I more lean towards individual ways). This also leads to the situation that roads joining from both sides are connected to this single way. I am no legal expert on Thai road laws, but I assume that it is not allowed to stop in the middle of the road on that "island" and do a right turn. Still this is what many drivers do in reality.
similar to this:

https://www.mapillary.com/app/?lat=18.7 … 836&zoom=0

https://www.mapillary.com/app/?lat=18.7 … ocus=photo

So would we have then to add a turn restriction to each joining road, as routing software is not aware of the painted island?
Similar to roads with a continuous middle line (or double line) which also prohibits crossing/turning.

I think it is important to have a good set of criteria when to tag a restriction. Otherwise we end up into discussions of how to interpret a situation and potential edit wars.

The initial example is easy, as there are signs.

Without the sign I would probably trat this like other u-turn points and do u-turns there if traffic permits.

Good navigation engines have a setting "avoid u-turns".

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