[OSM-talk] Changed highway=*_link meaning?!
gravitystorm at gmail.com
Thu Jun 24 14:26:14 BST 2010
On Thu, Jun 24, 2010 at 1:24 PM, Richard Mann
<richard.mann.westoxford at googlemail.com> wrote:
> On Thu, Jun 24, 2010 at 12:32 PM, Andy Allan <gravitystorm at gmail.com> wrote:
>> the root of the discussion seems to have no basis in
>> the tags, and seems entirely to be around rendering artefacts that you
> What purpose do the _link tags serve other than rendering?
They can be used by routers to give more accurate descriptions - e.g.
since we don't (yet) indicate junction priorities, it can be helpful
if you are on a *_link and going onto a * to announce it as "join the
main carriageway". If it was e.g. just highway=trunk for both, the
router wouldn't know you were on a slip road. Same for when you are
approaching an exit, it's a nice hint to the router that you aren't
following the main carriageway.
> If there's a serious reason for tag-to-higher then we can add an
> additional tag so people can record the status of what it links to
> (and then we can render it any way we like). But I can't think of a
> sensible reason for recording/using the higher status, except for
> motorways, so it just seems like it's been copied from motorway_link
> without thinking it through, is producing unintended results, and is
> therefore an error that needs to be corrected.
There might be some edge cases, such as the one you previously linked
to. But let's take this one near Cambridge:
I think the slip roads on/off the trunk road dual carriageway are
quite rightly tagged as trunk_link. It is very little different from
the case of a normal motorway junction. If you had to choose whether
those slip roads were part of the trunk road or of the secondary road
crossing over it, I would think most people would go for trunk. And I
suspect the facts on the ground would lean that way, when it comes to
resurfacing, signage, speed limits, lane width, type of tarmac and so
on, that the slip roads are more likely considered part of the trunk
As I say though, it's a well used and well established scheme, and we
should be very wary of changing it just because of some edge cases
where the rendering doesn't work correctly or where a particular
junction seems bizarrely tagged.
> If people have done that thinking through, and there's a genuine
> reason for tag-to-higher for non-motorway roads, then I'd love to hear
> about it. All the reaction so far seems to be a complaint about how I
> did it, rather than the substance of the matter.
I think few people have expressed whether or not they support your
views or oppose them, but certainly the main point of this discussion
is that should we want to change it you can't just change the wiki and
declare it done!
> Andy's made one of the few moderately serious points: it's confusing
> to treat them differently to motorway links. Not exactly a clincher,
> if it's wrong for other reasons.
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