Above it was said that the highway=trunk vs highway=primary
distinction is mostly for routing applications. But allowing a proper
rendering is also a main goal of the road tagging system.

While it's true that road class is useful for routing when there are
two alterate routes, a main reason to tag highways with a certain
class is to be able to render maps properly at different zoom levels.

When you are making a high-scale, low-zoom-level map of a large area
(say, the whole State of Alaska, all of England, or all of Australia),
you will want to only render highway=motorway + highway=trunk, because
showing all highway=primary would lead to rendering many smaller roads
which are not reasonable to show at that scale in most places.

In England, where these tags were developed, the distinction between
highway=trunk and highway=primary is subtle: both are "A" roads in the
official classification system, but highway=trunk has a special
sub-classification which says they are more important than other "A"
roads (tagged as primary): "UK OSM users follow the practice that all
green-signed A routes (ie primary routes) are tagged highway=trunk,
while black-and-white-signed A-roads (ie non-primary routes) are
tagged highway=primary".

Thus in the USA it's reasonable to use highway=primary for most State
and some US highways, while the most significant ones which connect
cities and large towns would be tagged highway=trunk.

Look at England at z7 on the Openstreetmap-carto style (the highest
level where highway=primary is not shown):

The highway=trunk roads shown here are the main routes between cities
and towns. Zooming in to z8 shows a dense network of highway=primary
roads connecting smaller towns and large villages to towns and cities,
which would not be reasonable to show at z7

Unfortunately, the road classification system in parts of Continental
Europe was different, so mappers in some major countries, including
Germany and France, chose to use highway=trunk as synonym for
"motorroad" (somewhat similar to a U.S.A. "expressway"), with other
major roads tagged as highway=primary. If you look around the
Openstreetmap-carto rendering of Europ at z7, you will see many gaps
in the rendered road network in these countries and surrounding areas
that use the same system.

Compare Spain and Romania, which instead use highway=trunk for all major non-
motorway roads between cities: here the country-wide road network is
clearly visible with showing just highway=trunk and highway=motorway
at z6 and z7.

In the USA, it's fine to limit highway=trunk to expressways in eastern
States where all the important US highways are expressways and these
form a dense network connecting all cities and towns. But in
sparsely-populated Western states even some of the Interstate highways
are not fully motorways, and almost all US highways are just 2 lanes
(one each way) in the area between the Cascades and the Rocky
Mountains, even those that are the main cross-State routes. If we
don't tag these highways as highway=trunk it isn't possible to render
this area in a reasonable way while using the same rendering rules for
the whole USA.

Major US and State highways between cities, like AK-2 and CA-199,
CA-299, US 97 (main route in Eastern Oregon) and US 101 should be
tagged as more significant than a tiny State road in Delaware which
only connects small towns and villages.

I would suggest looking at the Indonesian road tagging guidelines
(which I was not involved in developing, but I use in mapping
locally): they show very different road quality between the developed
areas and the remote parts of the country:
Most trunk roads are only 1 lane each way, but they are still the
main, National road connecting the large cities on each island. This
should be expected in other large countries like the USA, Australia
and Canada.

For tagging the status of a road as a "motorroad" or "expressway" I
would recommend using the tags motorroad=yes and expressway=yes,
rather than tagging all expressways and motorroads as highway=trunk no
matter their classification or significance in the road network. And
adding maxspeed=, surface=, lanes= and access=  will allow routing
applications and specialized renderers to treat these roads properly.

Joseph Eisenberg

On 12/21/19, Graeme Fitzpatrick <graemefi...@gmail.com> wrote:
> On Sat, 21 Dec 2019 at 08:53, Paul Allen <pla16...@gmail.com> wrote:
>> On Fri, 20 Dec 2019 at 22:05, Graeme Fitzpatrick <graemefi...@gmail.com>
>> wrote:
>>> But would they still count as either =trunk or =primary?
>>> While they're of high local importance, they're definitely not
>>> high-performance & they don't link major population centres either?
>> You have just identified three orthogonal dimensions:
>>    - Construction (what you call "performance": motorway or dirt track)
>>    - Traffic (number of vehicles per hour)
>>    - "Importance" (read on)
>> Thanks, Paul - I don't disagree with a word you said, except maybe the
> importance of road construction?
> I mentioned "performance" & "importance" as they are the definitions used
> for highway=trunk
> https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtrunk
> "Use highway <https://wiki.openstreetmap.org/wiki/Key:highway>=trunk for
> high performance or high importance roads that don't meet the requirement
> for motorway <https://wiki.openstreetmap.org/wiki/Tag:highway%3Dmotorway>.
> In different countries, either performance or importance is used as the
> defining criterion for trunk"
> While =primary refers to "A major highway linking large towns, in developed
> countries normally with 2 lanes. In areas with worse infrastructure road
> quality may be far worse"
> https://wiki.openstreetmap.org/wiki/Tag:highway%3Dprimary
>> If the only way of getting from A to B is along a particular road, it's
>> very important.
> In some of the cases that I'm thinking of, these are the only roads, so
> yes, very important.
>> Do we need to broaden our classes of construction/legality somewhat to
>> encompass
>> standards in countries outside the UK?  Probably.  Are the values for the
>> highway key
>> UK-centric and somewhat misleading?  Sure.
> Thank you!
>> Should we map dirt tracks between Hell's Bumhole and Arse-end Of Nowhere
>> as motorways because they're the only
>> way of getting from one place to the other?  No, no, a thousand times no.
> & yes, yes, a thousand times yes, :-) I agree that they shouldn't be
> =motorways, BUT, they should be either =trunk or =primary, because, in this
> area, they are the main road, & so are of vital importance!
>   Thanks
> Graeme

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