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

[Mike Parfitt]

Have a look at www.openstreetmap.org/changeset/79582073

On my Samsung Tablet I have been using Bing satellite images with the Vespucci editor, but other images also show the road markings quite clearly.
________________________________
From: Ed Loach <edloach at gmail.com>
Sent: 14 January 2020 13:45:34
To: 'Paul Berry' <pmberry2007 at gmail.com>; 'Mike Parfitt' <m_parfitt at hotmail.com>
Cc: talk-gb at openstreetmap.org <talk-gb at openstreetmap.org>
Subject: RE: [Talk-GB] Motorway junctions where the slow lane seperates from the through lanes


See also

https://wiki.openstreetmap.org/wiki/Lanes

which has some quite good notes on how to map lanes. I suspect this is how OsmAnd knows to give me lane guidance (can’t think how else it could know).



I suspect based on that you’d want to begin your new way for the drop lane where the lane splits away from the main carriageway, with an earlier split for lanes=3, turn:lanes=slight_left|through|through with lanes=1 on the new way and lanes=2 on the main way after the split.



Ed



From: Paul Berry
Sent: 14 January 2020 13:15
To: Mike Parfitt <m_parfitt at hotmail.com>
Cc: talk-gb at openstreetmap.org
Subject: Re: [Talk-GB] Motorway junctions where the slow lane seperates from the through lanes



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.



Regards,

Paul



On Tue, 14 Jan 2020 at 08:26, Mike Parfitt <m_parfitt at hotmail.com<mailto:m_parfitt at 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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