Handle the simplest sausage link case about osm2streets HOT 9 CLOSED

A google failed to help me define a sausage link intersection, do we intend to have a glossary in this repo?

Yeah, we probably should! I'm not sure where I heard the term first. https://labs.mapbox.com/mapping/mapping-for-navigation/implicit-restrictions/ mentions it. It's when a road constantly transitions between a single and dual carriageway due to some kind of intermittent physical barrier. When rendered in Carto (and osm2streets so far), the road looks kind of like sausage links

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Part of this will be fixing up the lanes inferred on each piece, because it's definitely wrong right now. It's possible I might block working on this on cutting over to osm2lanes finally, so any fixes can happen in one (better tested) place

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My attempt at something like this in the JOSM Lanes plugin was looking at one merged intersection at a time, but not at any graph properties. I am exploring that idea further, so it would be cool to see any ideas that other people have about it. Looking at the graph might be very powerful, for instances there's a little loop of oneways in the graph for each of the sausages.

It will be interesting for me to see you tackle a transformation in RawMap as a sort of template for how I might do it.

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(I would love to figure out how to properly model crossing islands like this, but that's a later step.)

(I think: the presence of a traffic island implies that a short stretch of the divider has a substantial width. i.e. traffic_calming=island or crossing:island=yes implies a median buffer (which I don't quite know how to tag yet).)

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It will be interesting for me to see you tackle a transformation in RawMap as a sort of template for how I might do it.

https://github.com/a-b-street/abstreet/blob/5907a8e3035ff7ae4af43568e5ad8d651d2fe400/raw_map/src/transform/find_short_roads.rs#L116 is one half of an example. (The other half is merge_short_road, which doesn't handle all edge cases)

(I think: the presence of a traffic island implies that a short stretch of the divider has a substantial width.

Definitely, and it'll be interesting to see if most roads keep the same overall width and shrink the lanes a bit near the island. There's likely a variety of ways to tag the island; this one example uses an area, https://www.openstreetmap.org/way/496700815, with some footpaths

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A google failed to help me define a sausage link intersection, do we intend to have a glossary in this repo?

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Need to detect and avoid false positives like:

https://www.openstreetmap.org/node/8141904208#map=19/51.49211/0.00378
https://www.openstreetmap.org/node/9563439140#map=19/51.49990/0.00457
https://www.openstreetmap.org/node/90266738#map=19/51.46112/0.03800
https://www.openstreetmap.org/node/90407230#map=19/51.47857/0.05173

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Section "Lane markings: Traffic islands require very special treatment" of https://strassenraumkarte.osm-berlin.org/posts/2021-12-31-micromap-update describes a similar process to this transformation

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I made progress with snippets of contraflow cycleway in the branch above. But I'm hesitant to enable this yet, because I had to shuffle around the order of transformations to make it work. We happen to collapse a short road after doing geometry, and only then do we have the graph structure to spot the sausage link. This is starting to feel precarious -- we could be brute-force and repeat a bunch of transformations until nothing changes anywhere. But is there a more principled and/or performant way of doing it?

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