The case was that many of those roads are created by continue. If the main road is merged but the direction dependant road not - then at lower resolutions the DP filter will create a mess. This is only relevant for 21 or lower I feel. 22-24 it's all right. Most overlays will be right in that resolution. Even though I have loads of those overlays, luckily those that are direction dependent are only in resolution 24 and 23. Some people may have overlays at lower resolution however.
All lines created with continue at resolution lower 23 are problematic concerning this matter. And because I have lots of overview - E.g. oneway arrows, oneway streets are loads, there is quite a difference in map size regarding oneway roads being merged from resolution 23 onwards or not. I think it makes up for around 1.5% map size. The 1.5% is not the problem, but that primary roads, highways and so on are usually mapped with two lines, and each one has oneway attached. Then quite a bit of the optimization is lost if those roads are not merged. As for me highways are not routable, I just delete the oneway in style before processing... Could skip that if non routable lines do not care for oneway. Same for railways (and railways as well are displayed to quite a low resolution. Fixed by deleting oneway for railway=* & highway!=*