Guidance for length of placement=transition segments

General talk Tagging general discussion
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At typical roadway merges and diverges, there are road segments where the driving direction is not parallel to the way itself, but there is a (small) angle in between. These segments can be tagged with placement=transition to indicate this (or, alternatively, placement:start=* and placement:end=* with different values). This is commonly useful for describing lane geometry at motorway-like junctions and transitions between single-carriageway and dual-carriageway sections.

However, I am wondering if some guidance on the desired length of the transition can be given, as this appears to be poorly documented and hence varying according to personal preference of whoever mapped a particular road:

  • The placement=transition wiki page suggests to match the length of the painted gore, with which I tend to agree except if the gore is very long.
  • @Allroads contacted me stating that he instead prefers to use short segments. His transitions are typically a couple of meters long at roughly 45 degrees.
  • The placement proposal shows examples with transitions of various lengths, including one extreme example where the transition segment is practically orthogonal to the driving direction to make it as short as possible.
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It’s quite individual style in effect. One complication is indeed when long gores may collapse into a solid line (distinguishable when the jurisdiction uses double solid lines), making it practically similar to change:lanes= (ignoring any legal technicalities).
I personally use a longer compromise, drawing a straight line through the last chevron. It usually preserves the shape better.
But definitely not any 90 degrees. That’s worst of both worlds. The better solution according to the other examples is to draw it across the gore.
In extension, it can be considered together with separation:*= explicitly Proposal:Separation - OpenStreetMap Wiki

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For sections of road that have a clear transition start and end the length is defined.

For the sections where this in not defined the length does not really matter, however shorter sections are preferable because if the section is so long that the off ramp starts to curve than that cannot be rendered in the placement=trantion part.

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