[Tagging] Tag ref on motorway_link

Johan C osmned at gmail.com
Tue Oct 23 20:10:55 GMT 2012

Well, Andrews suggestion is a bit problematic.

1. in reality, the motorway_links are a official part of the motorway. Just
look at the signs on the side of the motorway_link. Thus, it should be
possible to use ref=A15 for example on a motorway_link.
2. a motorway entry would be quite simple for a router, because the
prediction would always be correct: the entry always (?) leads to one
motorway with one ref
3. when a motorway_link is an exit, it should be possible to use the
junction number on this link, for example as junction:ref. Why?
a) the next way might be a tertiary road. A routing engine can't predict
the correct ref from this tertiary road
b) the motorway_link is used to store information on destination, lanes,
lanes:extensions etc. It would be illogical to forbid the junction number
being used as well.
c) it quite often will be impossible for any (present/future) routing
engine to always predict the right exit number based on the
motorway_junction ref. Imagine a more complicated junction. These may use
y-splits where one highway=motorway is split into two highway=motorways. Or
one highway=motorway_link is split into two highway=motorway_links. What
should the prediction for the junction number be? Left or right?
d) following b and c, this also means that the highway=motorway_junction
node is pretty useless for a routing engine. Only in simple exit situations
it will be sufficient to show the junction number in your navi-app

Bye, Johan

ps I have to complement Martin for his work to get pieces of the somewhat
messy OSM puzzle sorted out here: I read the Connect 11/2012 magazine which
tested several navi-apps, including Skobbler and Navfree. Connect is pretty
harsh on them, especially because of routing problems caused by

2012/10/23 Andrew Errington <erringtona at gmail.com>

> On Tue, 23 Oct 2012 14:47:40 Martin Vonwald (imagic) wrote:
> > Hi!
> >
> > I'm wondering what ref should be used on slip roads/ramps of a motorway
> > (not the junction node, but the way tagged with motorway_link). Up to now
> > I've seen: * the reference of the junction
> > * the reference of the motorway
> > * the reference of the junction not in the ref tag but in junction:ref
> > * nothing (neither ref nor junction:ref)
> >
> > In the Wiki (English and German) I couldn't find an answer.
> >
> > Any opinions/comments/hints are welcome.
> I suggest 'nothing'.
> The ref of the motorway can be inferred by looking at the end of the slip
> road
> where it connects to the motorway.
> Same for the junction number.
> A little bit of redundancy is good, but if we can reduce what we store
> (because it's easy to derive it from somewhere else) then there's no point
> in
> entering, storing, and maintaining it.
> Best wishes,
> Andrew
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