[OSM-talk] Removing redundant routing instructions

Greg Troxel gdt at ir.bbn.com
Sun Apr 26 13:57:55 UTC 2015

Rob Nickerson <rob.j.nickerson at gmail.com> writes:

> I'm not sure I get your point about "hint for router" versus "aid for
> navigation". I suspect this may stem from the don't tag for the renderer
> rule. If we look at the end use case the aim is to get a routing engine
> that provides an optimal route with user friendly route instructions. I
> can't believe this is an easy tag and as such I would expect the routing
> developers to be raising issues they cannot solve via code alone. This is
> one area where I would like my SatNav not to spew redundant instructions.

[not all directed at you of course]

I think the text of the rejected proposal could have been written
better.  I think declaring this a hint for the router is not really the
right characterization and led to trouble.  We're talking about encoding
facts about the world that are useful for multiple purposes. Also,
"don't tag for the renderer" is about not putting wrong tags in because
some renderer will make it look how you want.  Describing something
accurately so that it can be rendered is totally ok, and the main thing
we do.

What's really going on is that on the ground, it is (often) clear to a
driver what "continuing on this road" means, vs "turning".  This can be
because of signs, or because of subtle geometry of curb cuts, or the
widths of the continue vs turn roads, or various other clues.  Somtimes,
many of these clues can't be figured out from aerials, and certainly not
From road vectors.  So it makes sense to have a way to tag this so that
the map data captures this on-the-ground truth.  Often it's obvious from
the geometry (and the obvious answer is right), and those cases don't
need tags.  I think for now we should avoid getting into rules about
when it's not needed, and be ok with people adding the tags if they
think it's confusing.

I view this as similar to turn restrictions, except that instead of
telling you what you can't do, it's documenting what it means to not

I would suggest something like "turn_description=straight", or maybe
=none, to be used on the from/to ways at any junction where the
look-at-the-map-and-obvious-guess answer isn't right.  They should be
directional because perception is going to be different, and sometimes
you'll need both.

There's a further question about whether routers should announce.  I'd
say that if the road turns 90 degrees, it should, but it should say
"turn left to stay on Route 2" rather than nothing.   I guess this is
another wrinkle in tagging, and again is a fact about the world more
than a router kludge.  So I come down to

  turn_description=straight       (road continues, obvious to driver)
  turn_description=stay_on_road   (road continues, not obvious to driver)
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