Didn't Settlers III abandon the routing by roads and flagposts? The little guys still go from building to building, but you don't set any roads or route markers beyond that.
I haven't played any of the games after the third one so I don't know what kinds of changes they've made past that, but Settlers III definitely did away with the roads-and-flags routing.
I think you’re right. I’m looking at some screenshots and it looks like SIII don’t have any routes with posts. They still walk and “make their roads” but there are no posts. SIV seems to have moved from roads to “general loss of grass in busy areas”.
I haven't played any of the games after the third one so I don't know what kinds of changes they've made past that, but Settlers III definitely did away with the roads-and-flags routing.