Hi vas111, thanks for the query.

For intersections between dual and single carriageway I work off the recommendations on the (now archived it seems) Mapbox intersection modelling page (https://web.archive.org/web/20230602212339/https://labs.mapbox.com/mapping/mapping-for-navigation/modeling-intersections-for-map-navigation/). This dictates that a split in the carriageway should not occur at the point of intersection, as it can confuse routing software and produce unpredictable results. The recommendation in this instance would be to either continue the separate ways of Market Street to the north side of the junction and join them into a single carriageway post- intersection, or alternatively have the intersection occur between a single and a dual carriageway, by combining the Market Street carriageways on the south side. I opted for the latter, as there is a natural space on the south side of the intersection for this to occur, allowing the mapped "theoretical" ways to better represent real life, versus the former.

If a further U-turn relation is required then that's not too big an issue - it is after all just a rule that will block U-turn routing from occuring. But the recommended intersection layout has a big effect on compatibility with different routing software, so I'd argue it's the more important need here. I should add though that the Aberdeen roads are mapped to a high standard, so great work if you've been involved in this 😊