[Tagging] traffic_signals:direction=* vs. direction=*

Greg Troxel gdt at lexort.com
Sun Mar 26 00:15:54 UTC 2017

Tod Fitch <tod at fitchdesign.com> writes:

> Quoted sections have been edited down to only show parts I am responding to:
>> On Mar 22, 2017, at 5:37 AM, Greg Troxel <gdt at lexort.com> wrote:
>> For highway=traffic_signals, the normal situation is that it's on a
>> node, and affects all ways entering the node.  Or it's on a way and
>> affects both directions -- in my experience, when there are traffic
>> lights not at an intersection, they are always for both directions (even
>> though in theory they could be set up for one direction only).
> CalTrans has used semi-permanent traffic signals to control traffic
> flow over damaged road sections. A light at each side of the damaged
> area controls the sequentially one-way traffic through the damaged
> area that has been reduced to one lane operation. I’ve seen these in
> the Santa Cruz mountains, but the longest section of road I’ve seen
> controlled this way was on the main road to Yosemite Valley where the
> side of the mountain came down on the main roadway and the work
> around, in place for several years, was to put some pavement on the
> old single track railway grade on the other side of the river and put
> two bridges in place to get traffic across and back. It maybe still in
> place while but I’ve not been through there in several years. I know
> they took a long time to decide what to do about the still unstable
> mountain above the covered section of highway.
> So there are places with traffic signals away any intersections that
> control traffic going into a one lane section. I guess a smart router
> could guess that the transition of a road from two lanes to one lane
> would count as an intersection but that seems an error prone
> algorithm. Having a direction tag of some sort available does make
> tagging this situation possible.

You are totally correct.  I have seen two traffic signal sets near me,
controlling access to a one-lane bridge and a road where one lane is
washed out and the other alternates.  I had forgotten about these.

But, while we need a way to represent these, I think the notion that:

  stop not at a node is towards the node only, unless otherwise tagged
  to be both
  signals is both ways, unless tagged to be just one way

is relatively sound, in terms of striking a balance between mapping ease
and data consumers.
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