u-turns right after intersections?

[emiltin]

there are numerous places in osm where a street changes into two parallel one-way streets. often at intersections. like here:
http://www.openstreetmap.org/?lat=55.671981&lon=12.57939&zoom=18&layers=M

this is often in relation to intersections. in effect you have three way meeting: two oneways separated only by a small angle, and a bidirectional.

osrm is happy to enter one of the oneway streets, do a sharp u-turn, and leeave out the other oneway. often this means you're leaving an intersections, do a u-turn right outside the intersection, and then re-enter the intersection.

i'm not really sure this is a bug; but the maneuver certainly seems untraditional. especially since these situations often occur at large roads (since they're the one that are typically divided at intersections).

http://localhost:5000/viaroute&start=55.671663936787,12.578800734469&dest=55.671733515879,12.57862370868&z=18&output=json&jsonp=showResultsRoute&geomformat=cmp&instructions=true

[DennisOSRM]

This is not a bug but a missing turn restriction if it's not allowed.

[emiltin]

i guess you're right. it's just that there are so many of these places, and none of them has turn restrictions.

[DennisOSRM]

That's true. Unfortunately, there is no tool which makes use of turn
restrictions yet. You simply don't see it on a map, so people don't know
they are missing.

A useful routing engine on the other hand may make this visible and
motivate people to add more and more of these restrictions.

On 17.12.2011 18:23, Emil Tin wrote:

i guess you're right. it's just that there are so many of these places, and none of them has turn restrictions.

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[emiltin]

yeah. it's possible to add turn restriction with potlatch2, and you see a small icon when you're editing. but it's bit more complicated than editing ways.

but another way to view it is that turn restrictions should be added whenever there's a roadsign. the situation i described above never has roadsigns permitting the maneuvre, i guess because it's implied that it's not legal?

[emiltin]

generally, routing across intersections on bicycle seems to present some challenges; i often get long detours (not bugs in osrm, just following oneways, etc). in reality, people would dismount and push their bike across. i'll need to look more into this.

[DennisOSRM]

Depending on the country there don't have to be any signs. Often a
non-dashed line in the middle of the street suffices to indicate that
such a turn is illegal.

On 17.12.2011 18:37, Emil Tin wrote:

yeah. it's possible to add turn restriction with potlatch2, and you see a small icon when you're editing. but it's bit more complicated than editing ways.

but another way to view it is that turn restrictions should be added whenever there's a roadsign. the situation i described above never has roadsigns permitting the maneuvre, i guess because it's implied that it's not legal?

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[emiltin]

you're right. looking at google street view, this is certainly the case in this situation. interesting.
you're also right about the fact that a routing engine is a great way to discover these issues, and a motivation to get the osm data right - that's what we're doing!

[emiltin]

and yet; i have a sense that manually adding turn restrictions to every intersection that has a divider would be a lot of work. i will have to ponder this...

[emiltin]

osm proposal for using divider=* for tagging street divider lines. this often indicates where you cannot do u-turns or otherwise cross the middle line: http://wiki.openstreetmap.org/wiki/Proposed_features/Divided_road

the tags is not yet widely used: http://taginfo.openstreetmap.org/keys/divider#values