[Talk-GB] Motorway junctions where the slow lane seperates from the through lanes

Paul Jaggard paul at jaggard.net
Wed Jan 15 01:11:21 UTC 2020


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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