[Tagging] Classifying roads from Trunk to Tertiary and Unclassified

Paul Allen pla16021 at gmail.com
Sat Aug 10 12:14:16 UTC 2019

On Sat, 10 Aug 2019 at 09:27, Joseph Eisenberg <joseph.eisenberg at gmail.com>

However, in England where the tag originated, highway=trunk is used for the
> main,
non-motorway highways in the country.

Erm, no.  It's not like that.  Almost, but not quite.

There are A roads (known in OSM as primary routes) which are important
routes connecting
major population centres.  There are B roads (known in OSM as secondary
routes) which
have lower traffic densities than A routes and/or connect lesser population

"A" roads are not synonymous with trunks in the UK.  In the UK a trunk road
is more of an
A+ road rather than just an A road and is funded/maintained by national
government rather
than local government.  Ignoring motorways, all(?) UK trunks are A roads
but not all A roads
are trunks.  As far as motorists are concerned, trunks are
indistinguishable from A roads
in terms of signage.  Although a non-trunk A road usually does not have the
capacity of a
trunk A road there are cases where that is not true.

And then there are motorways.  In OSM-speak they might be called nullary
(nihilary?) routes.
"A" roads are level 1, B roads are level 2 and motorways are level 0.
Trunks are maybe level
0.9 or 0.75 or 0.99 or something like that - a little better than non-trunk
A roads, maybe.

See https://en.wikipedia.org/wiki/Roads_in_the_United_Kingdom#Classification

As can be seen by glancing at the rendering of
> England, these highway=trunk connect just about every place=town in
> England: https://www.openstreetmap.org/#map=6/53.021/-1.033

Assuming they have all been tagged correctly, of course.  That may not be
the case.

The idea is that one can determine the classification of highway based
> on what size of settlements it connects:
> trunk - connects cities to cities ("National Roads")
> primary - connects a town to a city or another town
> secondary - connects a village to a town/city or another village
> tertiary - connects a hamlet to a village/town or another hamlet
> unclassified - connect farms / isolated dwellings to a hamlet/vilage
> or another farm.

In general, I dislike it when different countries interpret the same tags
differently.  But
I also don't like the possibility that OSM will render routes in a way that
differs from
their official classifications.  Civil servants have examined actual
traffic statistics and
considered actual road construction (bends, constrictions, junctions, etc.)
to classify
certain routes in certain ways.  If they say that the shortest road between
town X and
town Y is a B road rather than an A road, they had a good  reason to do so
(I hope).

Your scheme also suffers another problem.  I can point you at a road which
the biggest town in my county to the biggest (only) city in my county.  It
passes through
a smaller town and a large number of hamlets.  Is it a primary (big town to
city) or
a secondary (hamlet to hamlet to hamlet)?  You may think it's obvious, but
there are
any number of circuitous routes connecting that big town to the city.  I
can come up
with a route that takes me the thirty miles from the big town to the city
by wandering
to the other end of the UK and back, but hat route DOES connect the two.
Equally, the
shortest route between X and Y could be a twisty, bendy thing whilst
there's an A
road and a B road that connect the two which are longer but faster.

This system is internally consistent and works well for rendering, as
> well as for routing.

Shortest route vs fastest route.  UK road designations take both factors
into account,
mainly the latter.  People eyeballing things are likely to come up with the

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