|barrier = retaining_wall|
|A structure that holds back soil or rock from a building, structure or area.|
|Rendering in openstreetmap-carto|
|Used on these elements|
|Tools for this tag|
Ais a structure that holds back soil or rock from a building, structure or area. Retaining walls prevent downslope movement or erosion and provide support for vertical or near-vertical grade changes.
Also consider one-sided cutting=*s if it lines a highway and isn't a man-made wall. They're more flexible (but slightly less accurate) if you're refining an established way which already has other aspects that rely on its direction, such as already being oneway=yes or cycleway:left=lane, that would need to change to incorporate barrier=retaining_wall's "right side is bottom, left side is top" convention.
How to map
barrier=retaining_wall behaves same as natural=cliff, that is no crossing as left-right are different heights. Lower side on right side of way direction, often rendered with dots, triangles or the like.
Renderers and editors might indicate the lower side of the retaining wall (the right side of the OSM way) with small triangles pointing downwards.