Hi Mike,

Interesting points and no easy answer I fear.

I think in mapping terms the midlines of each carriageway after the diverge
will look more like a upside-down Y and I tend to do a bit of smoothing to
make it look less abrupt. I think this is what you're getting at (apologies
if not). It's not dissimilar to the situation where a single-carriageway
road splits around an island: because the way is drawn as a line—not an
area—the carriageway split is always going to look more dramatic drawn that
way compared to the smooth continuous reality of what's on the ground.

In the situation of a lane drop don't forget to keep track of the
lanes=<num> in the keys.

It might be easier if you just go ahead and map as you see fit then post
the changeset link if you want further commentary.


On Tue, 14 Jan 2020 at 08:26, Mike Parfitt <m_parf...@hotmail.com> wrote:

> The technical term is a drop lane.  This might later intersect with a
> roundabout, join with another motorway or primary road etc.  Between
> junctions, a single way for each direction is commonplace.  At junctions,
> there are ways for the through lanes and for traffic exiting and entering
> the motorway.
> For example, on a 3-lane motorway with 3 lanes going in one direction and
> no junction anywhere near, the way would typically be placed along the
> centre of lane 2.
> However, when lane 1 is designated as a drop lane, what was being mapped
> as 1 way needs to split into 2 ways.
> The question is where ?
> There are various anticipatory changes in road markings well ahead of the
> physical separation of the asphalt, together with blue and white signs,
> some of which precede the first of the changes in road markings.
> In the case described above, my convention is to pick the start of the
> shorter dashes between the drop lane (1) and the through lanes (2 and 3).
> From then onwards, the way for the through lanes is mapped along the longer
> dashes dividing lanes 2 and 3, while the way for the drop lane is mapped
> along the centre of lane 1.
> Others do it differently.
> See "https://www.gov.uk/government/publications/traffic-signs-manual";
> from where you can download "Traffic signs manual chapter 5 road markings
> (2019)" which is a PDF.  Page 82 contains figure 7.7 and text documenting
> drop lane road markings.
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