[OSM-talk] Highway=trunk : harmonization between countries ?

Greg Troxel gdt at lexort.com
Tue Aug 22 12:40:13 UTC 2017

Lester Caine <lester at lsces.co.uk> writes:

>> I'm afraid that you can't use speed_limit : on small roads, the official
>> speed_limit is not signed but follows the default one (90km/h in France
>> even on a lanes=1.5 road !). 
> Single carriageway roads here are 60MPH out of town and 30MPH in town
> but I'm not sure OSMAND actually understands that anyway applying 20MPH
> limit to all secondary and tertiary roads? The routes I am talking about
> are tertiary roads with 40 and 50MPH speed limit sections ... I do add
> them in where I find them, but many were not showing in North Wales :(

I agree with Lester, basically.  In the US, a problem with OSMand is
applying the motorway speed limit to a motorway_link.  Technically
that's sort of true legally, but it leads to routes that get off and
back on again, which is not only crazy but wrong, because driving that
way takes longer.  People have asked to have foo_link be half the speed
of foo, and I don't know if that's happened.

Two points:

  Speed limit does not describe the speeds that reasonably responsible
  real people actually drive on roads.  The UK/IE notion of 60 mph on
  all roads out of village centers is one example.  Another is in the US
  where there are many roads signed 65 mph where traffic normally moves
  at 80 mph.  So, what I think OSM needs a few things:

    - A) a "typical_speed" tag, to be used by routers instead of

    - B) a project-curated worldwide  machine-readable ruleset that maps
      tags (including classifications, lanes, speed limit, and anything
      else) and possibley some geometry rules (distance between
      intersections) to the value "typical speed" should have, in cases
      where it isn't explicitly there.

    - C) perhaps a database of typical speeds, outside the map, that can be
      crowdsourced in some privacy-respecting way, that routers can use
      instead of A and B.

   Routing programs are often not quite right.  Routing is hard.  But I
   cannot agree with the notion that the way to fix routing is to change
   how we use highway tags and especially trunk.  Routers should be
   attempting to answer "if I took this route, how long would it take in
   both distance and time" as a prelude to finding a route with a
   minimal combined metric, and that's about many things and not really
   about classification. If a router really wants you to take a road
   solely because of classification, that seems like a bug.
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