Normally this occurs due to two or more parts with some overlapping edges. It’s a bad habit to do and can drive one to the conclusion that the builder wasn’t putting much effort into the work (which probably isn’t true because it’s only for half of them) or didn’t care about it.
The details on the map are spot on! Everything looks realistic as if it came out of a movie. Some things you could maybe consider is adding cracks in the walls because normally when plants start to over grow they can create cracks in the walls, roads, and other stuff.
Another thing to consider is adding water pipes underneath the parts of the road that have been broken down. When I was exploring I saw that you did this on some parts of the map but normally when a road breaks down the pipes break down as well.