Ok, so the milestone system is broke, we all know that it's hard to travel on it with a wagon without the damned thing being all zig-zaggy and whatnot. People have suggested other fixes, like being able to move through it, but that makes no sense, it's made of stone, and as such, is a physical object.
My suggestion is that when you place a milestone, instead of making a green radius, which is useless imo, build the milestone marker, then when attaching a sign, make the user select a location to place a "node".
The node would act like the green circle, and be within its radius, but instead of heading to the marker after leaving the previous, the user would be guided towards the node ON the marker. This could make using a wagon alot more stable, as the markers can all be on one side, and follow generally a straight line.
I'd include a pic of what i mean, but i'm useless at drawing.
basically: x=marker, o=node
x o
x o
x o
And the user moves towards the nodes from the markers as opposed to the markers themselves.