[Talk-us] Trunk

Kevin Kenny kevin.b.kenny+osm at gmail.com
Fri Oct 13 23:49:33 UTC 2017

On Fri, Oct 13, 2017 at 7:30 PM, Greg Troxel <gdt at lexort.com> wrote:
> I don't think "important connecting role in the long distance road
> network" should have anything to do with it.   A regular US highway that
> is not divided, grade-separated, mostly limited access is still a key
> interconnecting road, and it's squarely "primary".  Most of US 20 is
> like this, as I understand it, and all or almost all of the parts I've
> driven on (MA, WY) are like that.

You're saying basically the same thing I've been saying. But... people
who do routing and make maps are asking for three things, and
we separate them only incompletely.

(1) How "important" is this road to the long distance road network?
If you look at a small-scale map of the state of Maine, you'll virtually
always see US 1, 1A 2, 201; Maine 6, 11, 16, 161, 205. Maine
has only the one Interstate (95) plus a loop (295 from Portland to Augusta)
and a spur (395 into Bangor).

This is what guides the decision to render a road at a given scale.

(2) What are the road's physical characteristics (access control,
grade separation, number of lanes, width of shoulders, presence
or absence of traffic lights and stop signs)?

This is what guides the symbology to use. While Maine 205 is
an important road in its area, it is NOT rendered as a freeway,
or even a trunk. It's at best a primary and may even be a secondary,
and that's how is should be rendered even on a small scale

(3) How fast does traffic ordinarily flow on the road?

This is what (should) guide the routing decision; routing is
ordinarily done to save the driver's time. It is of key importance
to navigation systems, but doesn't ordinarily guide rendering.

In places with sensibly-designed road networks, these three
concerns are strongly correlated, and a road that scores high
on any axis is likely to score high on the other two. But this
is the US. Using the standing of a road on any of these
three axes as a surrogate for the other two is doomed to
a suboptimal result.

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