[Talk-us] Trunk

Bradley White theangrytomato at gmail.com
Sat Oct 14 02:10:48 UTC 2017

Lots of words ahead, you have been warned...

I disagree with trying to use the "highway=" tag to describe what
"kind" of road a given way is in the US, except for freeways. The
"highway" key is for importance, or, how prominently a road should
show on the map. We have other tags to describe completely the
physical attributes of a road. If there existed an "if-then" algorithm
to determine how prominently a road should show on the map only from
physical attributes, we wouldn't need this tag at all.

But we do. The U.S. needs this tag because form often does not follow
function. There are many plain-old two-lane roads that are important
enough for cross-country navigation that they should show at low zoom
along with the interstate system. There are also countless high-speed,
high-access-control (no driveways, limited intersections), multi-lane,
divided arteries that do little more than connect suburbs to more
important roads. These roads do NOT need to show at low zoom, despite
being a high-quality road.

Consider instead that we trade off "highway=motorway" with
"highway=1", "highway=trunk" with "highway=2", etc., which represents
only the importance of a road in the network. In the US, it is fair to
take freeways as an entire class to be the most important roads. A
freeway has strict & verifiable physical criteria (aside from a small
fringe set of exceptions), they are signposted and unambiguous, it is
a term well-understood in common vernacular that the average map
consumer would expect to see shown on a map, and they are nearly
always THE most important roads in the area. It is easy to say both
that "this is a freeway" and that "these are the most important kinds
of roads". In fact, this is the case in nearly all developed countries
on the planet. So, we instead define "highway=1" to just be
"highway=motorway" since it is nearly always the same thing.

In Europe, it would also be fair to use "highway=2" to simply
represent expressways as a class. Expressways (in the countries that
have expressway systems) are built to a verifiable design criteria,
are signposted and unambiguous (see:
https://en.wikipedia.org/wiki/Limited-access_road), are something the
average map consumer in the area is both familiar with and would
expect to see on a map, and are nearly always second in importance to
the motorway system. Again, it is easy to say both that "this is an
expressway" and "this is the second most important kind of roads". So,
in countries that have designated expressway systems, they define
"highway=2" to just be "highway=trunk" since it is nearly always the
same thing.

This situation is NOT the case for the majority of the US, and trying
to use this definition is what has been leading to unresolved
confusion about the purpose of the trunk tag. MUTCD gives a definition
of "divided highway with partial control of access". This is rather
vague, and as stated above, means countless unimportant suburban
arteries would now be considered the second most important roads in
the country. Many states haven't even adopted usage of this term at
all. I have seen planning documents of some counties that have
multiple grades of "expressways" depending on intersection distance,
speed limits, etc. Outside of bona-fide urban expressway systems like
the Santa Clara Expressway system, I think it's a nearly meaningless

I have heard two kinds of attempts to describe what constitutes an
expressway and thus a trunk road in the US. The first attempt is the
"it's like a freeway but only kind of" definition. I don't like this
definition because if we're going to trade off an entire class of
importance with an entire class of road design, we should at least be
perfectly clear about what kinds of roads we are talking about, as we
are with freeways. The second attempt is to establish some kind of
verifiable physical criteria: divided, minimum 45 mph speed limit,
limited intersections, maybe has grade-separated interchanges. There
are also many problems with this approach. As discussed above, it
grades swaths of overbuilt roads as being more important that they
actually are. It makes roads that are crucial for navigation but don't
meet an arbitrary checklist difficult to pick out from the sea of
primary roads that the rural US currently exhibits. Furthermore, it
leads to the current situation of random splotches of deep-orange
lines visible at the same level as the interstate system scattered
across the US, which provides absolutely no meaning to the average
US-based map consumer. Being frank, I can't understand at all why
anyone considers the current state of trunk road tagging in rural
parts of the US desirable or useful or illuminating at all.

I propose defining trunk in the US to mean, formally, "The most
important non-motorway roads in the country". An extended definition
for the US follows below:

Trunk roads are the most important non-motorway roads in the country.
These roads connect major population centers and are crucial to the
transportation network as a whole, acting as a supplement to the
interstate system for cross-country travel. They are usually US
highways, but may be of other designation as well.

In rural areas, these roads often - but not always - are built with
expressway features (divided carriageway, access control,
grade-separated interchanges) at some or all portions of the road.
Often, these roads will circumvent towns along the route, but they may
also penetrate towns as an important road ("Main Street"s), in which
case it is to remain trunk.

In urban areas, most expressway systems should be tagged trunk,
especially if they serve as an important artery through the city not
already served by a freeway. If a trunk road enters from rural into
urban, in general it should remain trunk until it connects with a more
important road (motorway), or passes through the city completely (as
an exception, cases where the city IS the important destination may
end the trunk designation at the city). Very minor expressway systems
that serve small population centers or destinations should NOT be
tagged as trunk; instead consider using a lower classification of road
along with "motorroad=".

I like the ideal of only tagging what's on the ground, but as stated
before, this is something we're going to have to be pragmatic and
realistic about. If the expressway definition worked in the US, we
wouldn't be having unresolved debate about it for years on end. I
think it's time we make "expressway" a footnote of the "trunk"
definition, rather than hinging the entire tag on a vague and often
useless concept.

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