[OSM-talk] Clarifying and representing road markings at junctions

[Blaž Lorger]

On Tuesday 29 September 2009 21:59:10 Matt Williams wrote:
> I've been noticing recently a problem we're going to/already have in
> our data when it comes to routing directions particularly. It concerns
> how to define continuations of roads at junctions and/or the road
> markings that delineate that. This problem manifests itself in many
> ways but for a first example, look at the attached image (road.png).
> 
> On the left you will see the physical plan of a road junction near
> where I live. The way that it would be represented in the OSM data
> model is shown on the right. In this case, it would be sensible to
> make a way out of the segments 'a' and 'b' (yeah, I know we don't have
> segments any more, it's just an explanation tool), call it, e.g.
> 'Curve Road' and make a second way out of segment 'c' and call it
> 'Small Road'. At this stage, the date representation is sound and
> routing application would have no problem knowing how to parse it.
> However, there are two (increasingly common) ways in which this model
> will be forced to be broken:

I don't see where problem lies.

Is it that routing software will not be able to choose right route?
You never stated it clearly, but if I understand correctly road from segment a 
to b has right of way over segment c. So all you need is a way to indicate 
this. There are some proposals that could solve this (stop signs, yield, right 
of way).
If there are turn restrictions you can map those using turn restriction 
relation.
But if there is no explicit right of way and no turn restrictions it really 
should not matter how road markings are painted on the road. Routing software 
should be able to pick the right route based on other criteria (road 
classification, speed limits, traffic calming, ...).
Of course if roads b and c lead to completely different destinations (they 
don't join for several kilometers) it should be really easy to pick right way 
for specific destination.

The other problem could be that routing software will not be able to properly 
guide you through the junction.
If you take care that geometry of junction is represented correctly routing 
software will be able to guide you through the junction correctly. At least 
graphical representation should be correct.
Question is, will (voice) instructions be correct? I guess that in such 
situation clever navigation software would avoid using instruction 'go 
straight', but would rather use instructions 'keep left' or 'keep right'.

 Blaz