[Talk-GB] traffic island mapping / harmful detail?

[Chris Hodges]

There can't possibly be a blanket right answer given the variety that 
exists.


Here are a couple of edge cases, mapped differently with the difference 
perhaps the wrong way round. I suspect both were built as 3-lane single 
carriageways, and when the suicide lane was taken out (in one case for a 
blind summit, in the other for residential development) a physical 
barrier was put in.

A432 near Coalpit Heath - a short (approx 100m) stretch mapped like a 
dual carriageway :

https://www.openstreetmap.org/search?query=coalpit+heath#map=18/51.51764/-2.48006

https://www.mapillary.com/app/?lat=51.517289928208584&lng=-2.4821660108887045&z=17&mapStyle=OpenStreetMap&pKey=filwUHxY-F4va7l0rPe3hg


A48 through Caerwent (a truly evil piece of infrastructure that ends up 
not quite wide enough to overtake bikes, but looks like it is in some 
people's eyes). This is mapped as single carriageway despite having an 
island/central reservation for 700m with breaks for junctions, some long 
enough to allow overtaking:

https://www.openstreetmap.org/search?query=caerwent#map=16/51.6113/-2.7636

https://www.mapillary.com/app/?lat=51.613820660907436&lng=-2.7701350953456085&z=17&mapStyle=OpenStreetMap&focus=map&pKey=ms5KN9FNkcPVbeMshf-tXA&x=0.48650452832085844&y=0.6375408554628968&zoom=0

The Caerwent one has a couple of explicit crossings at the lights, but 
people cross anywhere.  The Coalpit Heath one is used for crossing but 
the pavements aren't busy round there and there's little need to cross; 
normal islands are provided further SW near the next junction).


Routers have comparable trouble to renderers in that you end up having 
to put your waypoint on he right side of the road  or get spurious 
directions/wiggles/distance.  That in itself isn't persuasive of course.


Considering the pedestrian user, adding an island should mean crossing 
is possible at that point, a split alone doesn't though it can be tagged 
- but that needs to be done with care: some (even slow) dual carriageway 
reservations are built to make crossing hard using stuff like high kerbs 
with ankle-turning stones set into them ( 
https://www.mapillary.com/app/?lat=51.483120471287066&lng=-3.170204088094806&z=17&mapStyle=OpenStreetMap&focus=map&pKey=PpuE01NBZCQvviv-dsgOrw&x=0.5138150658353747&y=0.637737650819708&zoom=0 
is  used as a crossing in practice, by the agile) or partial fences/planting




On 31/03/2021 12:54, Mark Goodge wrote:
>
>
> On 31/03/2021 12:30, Stephen Colebourne wrote:
>> I add some areas in the middle of traffic, for pedestrians.
>>
>> My rule is that I add them in association with explicit sidewalks, as
>> it makes it much easier to follow the sidewalk. The classic case is
>> where a pedestrian crossing is two-phase, and the pedestrian has to
>> walk parallel to the road, as shown here (I hope it can be seen that
>> the map is clearer for pedestrians with the island explicitly mapped):
>> https://www.openstreetmap.org/#map=19/51.40863/-0.21544
>>
>> I wouldn't split a road to do this though - I'd be adding it in a
>> place where the road already justifies being split. For a small
>> pedestrian crossing island I feel that `crossing:island=yes` is the
>> best option (and I always add `crossing:island=no` too):
>> https://www.openstreetmap.org/node/7259595189
>
> I entirely agree with this. Splitting a road just to add street 
> furniture is, IMO, the wrong choice. For a start, it looks odd on the 
> standard user-facing map (yes, I know, don't tag for the render, 
> but...), which I think is unhelpful. But, also, it goes against the 
> way that roads, at least in the UK, are classified in real life.
>
> Splitting a road creates a short (very short!) stretch of dual 
> carriageway as far as the map is concerned, but as far as highway 
> legislation and management are concerned the presence of an island 
> does not, alone, create a dual carriageway. It only becomes a dual 
> carriageway, and therefore should only be mapped as a dual 
> carriageway, if there is a median between the two halves that's long 
> enough to form a distinct pair of matching carriageways. The minimum 
> length necessary to create this is a valid matter of debate, but I 
> think Stephen's first example, above, clearly does meet those criteria 
> whereas the presence of an island at the second does not.
>
> Mark
>
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