Tag:placement=transition
 placement =
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Used to indicate that the shape of the OSM way follows neither the middle nor the left/right side of any lane. 
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| Group: properties |
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| Status: in use |
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The tag placement= can be used to indicate that the shape of the OSM way follows neither the middle nor the left/right side of any lane. This value is usually only necessary if the road forks or joins with another one.
Tagging example
A good example would be a motorway exit, see above. Start the way segment that is in transition where it departs from the highway at the painted gore, end it at the start of the physical separation (This would be called merging at the theoretical gore). This is visualized in JOSM as the selected (red) segment in Example 1. This may result in a steep angle: This is definitely a compromise between aesthetic consideration and what data consumers need to provide road users with navigation that accurately reflects decision points.
placement:start=* and placement:end=* were mentioned in the original proposal as an alternative tagging scheme to placement=. This pair of keys allows specifying the transition in more explicit detail. Note this is not possible if the start/end placement lies 'outside the carriageway'. In the above example, placement:end would be middle_of:1, but placement:start cannot be specified as it lies beyond the right side of the rightmost lane. Conversely, placement= only indicates that placement:start and placement:end are different, but not how.
Possible tagging mistake
Do not use placement= to merely indicate that some lane's width increases or decreases along the way (for this purpose, consider using width:lanes:start=* and width:lanes:end=* instead). placement= should only be used if the way does not align with any lane, i.e. if placement:start and placement:end would differ.
Original proposal
See the proposal for details.