Hi
I would suggest that the motorway should not be split until the point
where the two halves physically diverge; instead, where there's a drop
lane, use turn:lanes and destination:lanes tags to indicatethe presence
of the drop lane.
My reasoning for this:
- firstly, there's no physical separation between the drop lane and the
main line of the motorway, and it's still physically and legally
possible to change between the two, and
- secondly, splitting a motorway based on different lanes having
different destinations sets an awkward pattern, which taken to extremes
means we end up splitting roads all over the place based on lane
markings. Sometimes that's necessary to make routing work sensibly
through complex junctions, but I'd argue that this isn't one of those cases.
Best illustrated with examples:
Drop lane starts here. Before this point, a single way with lanes=3.
After this point, still a single way, with lanes=3,
turn:lanes=slight_left|through|through :
https://www.mapillary.com/app/?pKey=v4H-uhxh9QQYjzL1b_BNMg&focus=photo
The drop lane then runs for 300 metres, where there's no physical (or
legal) barrier between the lanes, so still a single way with lanes=3,
turn:lanes=slight_left|through|through :
https://www.mapillary.com/app/?pKey=S-dGYQdsR7olODRufbB5bg&focus=photo
Then the actual split, where first there's a legal barrier (solid white
paint), then a physical one (grass, barrier, trees, hillside, etc):
https://www.mapillary.com/app/?pKey=1WY5HbP5K-vpUnAT0voYGw&focus=photo
It's here where I'd suggest we'd split into highway=motorway_link,
lanes=1 (lanes=2 shortly thereafter), and highway=motorway, lanes=2,
with a shortish Y-shaped transition to make it look sensible. Of
course, at this point the main line of the motorway loses the turn:lanes
tag, and you can then add turn:lanes tags to the slip road as it widens
out for the roundabout at the end. The middle point of the Y (and not
the start of the drop lane) carries the highway=motorway_junction and
associated tags.
You could also add destination:lanes tags to label up the signed
destinations, and destination:ref:lanes=A4174|M32|M32.
Incidentally, for me, /this/ is the sort of situation justifying
separate, parallel ways on a motorway:
https://www.mapillary.com/app/?pKey=AL0RC-DyWe27PMQEAS1uZA&focus=photo
TL;DR? Don't split based on lane markings, it's not a 300-metre-long
physically separate way, it's just a lane with a different destination.
Instead use turn:lanes, destination:lanes and destination:ref:lanes tags
to indicate the drop lane.
Cheers,
Paul ("southglos")
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