[Tagging] Carriageway divider

Philip Barnes phil at trigpoint.me.uk
Sun Aug 26 22:00:36 BST 2012

On Sun, 2012-08-26 at 20:30 +0200, Erik Johansson wrote:
> On Sun, Aug 26, 2012 at 2:00 PM, Craig Wallace <craigw84 at fastmail.fm> wrote:
> > On 26/08/2012 08:42, Markus Lindholm wrote:
> >>
> >> Also, no one has offered any other solution to the routing issue. The
> >> divider tag has been proposed, but I think it has been demonstrated
> >> not to work, as routing decision are made on the node and not on the
> >> line.
> >
> >
> > Where has it been demonstrated not to work? What do you mean by "routing
> > decision are made on the node and not on the line"?
> > Yes, the divider tag is probably not supported by any current routing
> > software. But it would not be too hard to modify the software to allow for
> > it.
> This is an area which has two ways instead of one:
> http://osm.org/go/0bCzT1kfr--
> Here is an route example:
> http://map.project-osrm.org/1cL
> Without two ways you would be routed directly to the end point, but
> with two ways you will be routed with the needed detour. It's
> especially interesting if you go by bike but then it's more of a
> psychological divider than a physical (which is another story).
> I don't like mapping like this, but I'm pragmatic and it does solve a
> real problem, so I decided no to fight Markus on this.
> So what is the recommendation for mapping this.
Its a difficult question.

The pragmatic approach I would take is the driver should follow the
rules and let the satnav catch up.

I must admit I have cheated here, and looked at streetview. The U-turn
that the router has come up with looks very wrong to me, the road just
isn't wide enough to complete the turn from the left lane. To do a
U-turn from the right lane, across straight ahead traffic is dangerous.

In this case the false dual carriageway seems wrong and gives a wrong
impression of what is on the ground.

I think the correct and safe answer is this

Outside of a built up area, using this method will give the false
impression of the existence of dual carriageway and anyone using OSM as
a map will expect a fast road and will find a slow one where overtaking
is impossible.


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