As you can see there is a roundabout, but there is also a dual carriageway through the middle with the flow controlled by traffic lights. If you are in the lanes which go through as the dual carriageway you can't turn onto the roundabout, and if you are in the lanes that lead onto the roundabout you can't turn onto the dual carriageway lanes. I think for the relations to work I might have to split the roundabout from a closed way into a number of sections, but I've always drawn roundabouts as closed ways before. Will splitting it cause issues with anything else? Although I might not have to split as the via (node) will indicate where the restriction actually applies, even when the dual carriageway lane crosses the roundabout at two nodes. Or perhaps I could just split the dual carriageway lanes somewhere in the middle of the roundabout to make things easier for me to work out what restrictions I need to add.
I haven't thought this through, but I wonder if you could add relations to describe road groups, putting all the dual carriageway ways in one relation, and all the roundabout and link ways in another, and then adding a turns_prohibited restriction between those two relations. This seems pleasing since there is logically one extra rule to explain to humans - no turning between the roundabout and the dual carriageways that cross it. Of course this nested relation/restriction complicates routing software but perhaps not too badly.
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