[Tagging] works_as_highway=primary

moltonel 3x Combo moltonel at gmail.com
Wed Jul 29 11:11:34 UTC 2015

On 29/07/2015, Marc Gemis <marc.gemis at gmail.com> wrote:
> On Tue, Jul 28, 2015 at 5:29 PM, moltonel 3x Combo <moltonel at gmail.com>
> wrote:
>> A router won't care about classification differences between far away
>> places like Germany to Ethiopia. They just care about taking the best
>> road in the area, and as long as OSM is locally consistent, this
>> works. Even if a trunk turns into a primary for no physical reason and
> Seems like I didn't make my point correctly.
> I was trying to ask for global consistency so the router can use the same
> default weights for street types, everywhere in the world.  Something like
> 'prefer primary roads over secondary' roads  to travel large distances.

Routers can already use 'prefer primary to secondary' worldwide.
Nowhere in the OSM world is secondary defined as better than primary.
In any given area. a car router can confidently prefer 'primary'.

What's true is that 'primary is X times better than secondary' will
have different X values from one place to the next. But the
differences between section of a given road can already be more
important than the average difference between primary and secondary
(for example an Irish secondary´s maxspeed can go from 60 to 100, but
a primary isn´t generally 1.6x better than a secondary).

Consider also the case of motorways : in all countries I've driven in
they are very clearly defined and have legal specificities. OSM
couldn't afford to mistake a motoway with something else. Yet the
difference between a motorway and the next best thing is bigger in
Germany than in Ireland.

TD;DR: It's naive to think that routers can make a good decision using
the highway tag alone. Harmonising highway tag worldwide would be of
little use, and it would break local expectations. A locally coherent
highway tag is preferable, and if you want more precise routing add
the other tags.

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