How would I map these crossings?
The horizontal road is a single road, with a large median and each direction mapped separately. Currently, its mapped with a seperate crossing over each direction of the road, for six in total. Should the crossings be joined? If not, how does the tagging work?
The way this is mapped, with a separate crossing over each direction of the road, looks correct. Especially because the sidewalks have been mapped separately from the road.
The wiki uses these examples for traffic signals:
https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtraffic_signals
I would not join the 6 separate crossings into 1. Just tag each crossing as
highway=crossingand, if they exist,crossing=traffic_signals.I guess in that case, a crossing island between each direction would be tagged on both, if the island is not mapped separately?
https://wiki.openstreetmap.org/wiki/Key:crossing:island
It's hard to tell without some aerial/satellite imagery of your intersection, but assuming there are islands at each crossing, and assuming that "up" is North in your image:
The crossings at the Northwest and Southeast would be tagged as shown at the top of the page you linked:
While the 4 Northeast and Southwest crossings would be tagged as shown at the bottom of the page you linked: