[Tagging] Feature Proposal - RFC - footway=link

[Markus]

On Fri, 6 Dec 2019 at 03:30, Nick Bolten <nbolten at gmail.com> wrote:
> > I doubt that this were very useful, but would only complicate mapping. If you want to know the precise location where the footpath begins or ends, it should be possible to get that information from the road width or area:highway=*.
>
> Neither of these can reliably determine where the footpath begins or ends, as the footpath may end before the street begins, such as when there's a sidewalk between them.

If there's a sidewalk, it can also be mapped as an area
(area:highway=footway + footway=sidewalk). This way, it can be
determined where the footpath begins.

Alternatively, you could also tag the sidewalk width
(sidewalk[:left/right]:width=*), but this doesn't work well when the
width changes often.

> Let's use a nearby example: https://www.openstreetmap.org/way/86205950. This is a highway=footway, footway=link connecting a set of stairs to the street network, and it does so directly:  a highway=steps --> footway=link --> highway=tertiary. A pedestrian router would want to encounter these decision points: (1) continue onto the sidewalk vs. cross the sidewalk, (2) after crossing the sidewalk, potentially encounter a curb, (3) enter the street. Knowing the exact portion that is on the street is also useful, as that distance should not factor into the route - pedestrians generally stick to the side of a street when moving along it.

This example is quite different from the others as the steps run
parallel to the road and the sidewalk. (For a better visualisation,
i've mapped the sidewalk as an area, but you can also move about 100 m
southward, select the "Stadt Bern 10cm (2016)" aerial imagery and then
move back.) Therefore, you cannot continue the steps and connect them
with the road without breaking the orientation of the steps. The same
problem occurs with ending sidewalks [1] or with cycle lanes that
continue on the sidewalk [2], but also with crossroads with bollards
[3] or with an interrupted carriageway separator [4]. In these
examples, it makes sense to inform data users that these aren't
physical paths or roads, but merely connections needed for routing
purposes.

[1]: https://www.openstreetmap.org/way/554343046
[2]: https://www.openstreetmap.org/way/518400616
[3]: https://www.openstreetmap.org/way/749375744
[4]: https://www.openstreetmap.org/way/609505844

On the other hand, the part of the highway=path inside the road area
in the other example [5] is just an extension of the path to the
centre of the road. The orientation of the path doesn't need to be
broken in order to connect it with the road. Again, the
area:highway=tertiary makes it clear where the path really begins.
Therefore, i think mapping a path=link wouldn't be of much use here.

[5]: https://www.openstreetmap.org/node/413988097

Where i'm unsure whether a =link would make sense are steps that end
in a road, path or sidewalk [6]: on the one hand the orientation of
the steps wouldn't break when directly connecting them with the centre
of the road, on the other hand one would get a wrong impression of the
location of the first step and tread depth.

[6]: https://www.openstreetmap.org/way/416303537

> > Besides, only defining footway links, but e.g. not connections of tracks with roads (example [1]) or of roads with other roads (example [2]) seems quite arbitrary.
>
> The difference is that for vehicular traffic, the transition is correctly described: directly from track to road. It is spatially a bit fuzzy, but I've never encountered a routing scenario where a difference of a few meters of track vs. road were considered important for vehicular routing. The link schema would still be appropriate in those scenarios - it just needs to be valuable.

Tracks (as well as other roads not primarily designated for
pedestrians) can often also be used by pedestrians. In the linked
example [7], the track is even part of a hiking route.

[7]: https://www.openstreetmap.org/node/893450790

> > That short way definitely isn't a separate sidewalk (no lateral kerb, not parallel to the road), but the extension of the crosswalk.
>
> I'm going to politely push back on this. We have to draw this connecting segment regardless of whether the street crossing is marked (crosswalk) or unmarked (no crosswalk) and whether there's a curb ramp or not, so it's not necessarily part of a crosswalk. Even if this idea is amended to say that this short way is an extension of a street crossing regardless of marking, there's no appropriate crossing tag for it: is it crossing=uncontrolled/marked, crossing=unmarked, or unset? All of them are inaccurate in some way or another, or ambiguous.

I agree that the part from the centre of the sidewalk to the kerb is a
part of the sidewalk, but tagging it footway=sidewalk feels as wrong
as tagging a crosswalk highway=residential/... just because it is a
part of the road (or tagging the part of a driveway that is inside the
road area highway=residential/...).

Best regards

Markus