Just for reference. I found the old thread: http://www.mail-archive.com/mkgmap-dev@lists.mkgmap.org.uk/msg03680.html
Thanks for searching the old thread. It is true, now I can remember too. The problem was (and I think is) that the routing tables are generated before the merging. So the displayed lines does not match the rounting and pop up data. The solution would be to do the merging before creating the routing tables. I would expect it to have some small positive effect to the routing data too. But with the given structure of the code it is not easy to implement.
So if there is no way to correct it right (merge-lines before routing layer for resolution 24) - then I would support dropping merge-lines for resolution 24 and 22 by default (the error at 20 or 21 is much less apparent).
I'm sorry, but I will not have time the next weeks to investigate deeper. As I have read from Steves comment, the merge line code should not be called at resolution 24. I would have to check again, but I think this was the solution for the old thread. Not merging the lines at resolution 24 does not lead to discrepancies to the routing data. So it should be checked, why this code is executed at resolution 24. Johann