merge-lines is as discussed quite problematic, because it happens after the routing notes are placed, and therefore causes some problems. The closer you zoom in, the bigger these problems, while the further you zoom out, the bigger the speed improvement of using this option (at resolution 24 and 23 it makes basically no difference) and the smaller the problems (22 vs 24 means 1/4 of the wrong distance for info popup or jumping map when panning). There was a discussion about this - archive here: http://www.mail-archive.com/mkgmap-dev@lists.mkgmap.org.uk/msg07262.html This patch is working pretty well and makes --merge-lines usable. I have been using it since back then without any probs. So for as long as merge-lines cannot be implemented before creation of the routing nodes (which according to the discussion is really difficult) this tiny patch is great.