Take this hypothetical intersection for example:

Map of a hypothetical intersection

To turn left, one has to use the link road, and as such turning at the node where the streets cross isn't permitted. However, this in no way means that turning left isn't allowed at the intersection, and no prohibiting signs exist. (The same goes for U-turns.)

Should turn restrictions be added as in the picture? This would more closely reflect the functional reality, but I wonder if it might confuse human users who aren't expecting to see such symbols on the map. Is there a tag to note that a restriction is purely functional and shouldn't have a sign rendered?

asked 26 Jul '15, 20:37

Paul_012's gravatar image

Paul_012
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edited 27 Jul '15, 10:25

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sleske
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To turn left, one has to use the link road, and as such turning at the node where the streets cross isn't permitted. However, this in no way means that turning left isn't allowed at the intersection, and no prohibiting signs exist. (The same goes for U-turns.)

The import question is: "Is it legal to make a left turn here?" If "no", then you should add a turn restriction. If "yes", then you should not.

Whether it's legal depends on many things: (i) Is there a sign? (ii) Is there lane road markings that essentially means "This lane is only for straight on" (you can add turning lane tags too) (iii) Some other local law that makes a left turn here illegal.

I wonder if it might confuse human users who aren't expecting to see such symbols on the map.

Don't worry about that. That's the job of the renderers. The tag is added to mean "You can't turn left here", not "There is a no left turn sign here".

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answered 27 Jul '15, 13:24

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rorym
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JOSM with the turn restriction plug-in will show the restrictions as in your image. However I haven't seen a general purpose map display using OSM data that shows that. So it should not confuse any human users who are just looking at the map.

The turn restriction relations are functional in that routing engines are aware of them and they should assure the routing generated is compatible with any turn restrictions. In the case you show, it may not make much difference as using the turn link road should be shorter and thus faster than the other way. Unless, of course the maxspeed tags (or assumptions if no tagging) is off.

So adding turn restriction relations should only be a good thing. Assuming, of course, that you are using an editor that makes them easy to create and edit. Not sure about iD or Potlatch, but JOSM with the turn restriction plug-in is fairly reasonable.

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answered 26 Jul '15, 20:56

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stf
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sometimes the position of white lines makes it impossible to make a legal turn. I add turn restrictions in such cases as well, even when there is no traffic sign. And see my remark below for the cases where the drives missed the link road: should the navi then still say "turn left" after the link road ?

(27 Jul '15, 07:01) escada

is http://map.comlu.com/ the map you are looking for ?

(27 Jul '15, 07:09) escada

On balance I probably wouldn't add these turn restrictions to the map particularly as you say "However, this in no way means that turning left isn't allowed at the intersection and no prohibiting signs exist." – to me that is an indication the there are no actual turn restrictions.

I may be wrong but here is my reasoning …

Turn restrictions are added for routing software rather than for visual users of the map. If the routing software is doing its job properly it should (as already indicated in stf's answer) automatically route any left turning traffic along the link roads rather than making a turn at the nodes where the streets cross. In the event that a driver missed the 'link road left turn' the routing software should alert him to the possibility of doing a left turn at the nodes where the streets cross – provided the routing software could react quickly enough.

In real life, if the police were to see someone making a left turn where the streets cross they may consider it an unsafe (but not necessarily illegal) manoeuvre and issue a penalty notice.

One question about your hypothetical junction….. are there are lane marking arrows on the roadway? If there are arrows, but not including a left turn arrow, that could imply a 'no left turn restriction' but this may depend on local road rules.

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answered 26 Jul '15, 22:55

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But what if you missed the link road, should the navigation software then immediately say "turn left"after the link road (assuming it has the time to determined you missed the link road and could recalculate the route) ?

(27 Jul '15, 06:56) escada
1

escada...That was what I was trying to indicate in my answer. If there were no actual turn restrictions at the 'crossroad nodes' it would be appropriate for navigation software to say "turn left" at those nodes. However, I doubt if any navigation software would react/recalculate quickly enough for a driver who missed the link road turn.

(27 Jul '15, 08:37) NZGraham

sorry, I must have overlooked that the first time

(27 Jul '15, 11:06) escada
1

It's hypothetical, so I hadn't thought of lane-marking arrows. They'd likely exist where I live, so I can kind of see the merit of adding the restrictions. Judging from the answers here, I'd probably mark it as a low-priority task, for when there's nothing else to map.

(27 Jul '15, 12:55) Paul_012
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Hi Paul, I think the turn restrictions in your case are redundant. Unless of course there is a posted restriction that makes such turns illegal. Otherwise I don't see the benefit of taking the time to add them with JOSM or any other editor.

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answered 26 Jul '15, 21:15

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Why are the restrictions redundant? If the turn restriction on the main intersection is not marked, there is a danger that routing software will let you turn there (e.g. if you missed the link road).

(27 Jul '15, 11:12) sleske
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question asked: 26 Jul '15, 20:37

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