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OSM describes roads as center-lines that intersect. Turn these into road and intersection polygons roughly by
- treating the road as a PolyLine with a width, so that it has a left and right edge
- finding the places where the edges of different roads intersect
- “Trimming back” the center lines to avoid the overlap
- Producing a polygon for the intersection itsef
I wrote a novella about this: https://a-b-street.github.io/docs/tech/map/geometry/index.html