Some osmers who work fixing errors in OSM data using the files I serve have reported a lot of cases of supposed similar arcs which are not errors, like the following examples: (http://www.openstreetmap.org/browse/way/47972506) and (http://www.openstreetmap.org/browse/way/21690948) (http://www.openstreetmap.org/browse/way/152931346) and (http://www.openstreetmap.org/browse/way/327519978) (http://www.openstreetmap.org/browse/way/163137843) and (http://www.openstreetmap.org/browse/way/297772507) According to an old message from Mark Burton [1], ways with same start point, end point and length are reported as similar arcs by --report-similar-arcs option, even if they don't overlap. The ones above may meet those conditions, but it would be nice if not overlapping ways are omitted. Perhaps an extra check for a third point in common would help. [1] http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2010q1/006709.html