I modified the Road node lua file slightly.
Open file: Raod_node.lua
Add on line 26
group:AddProperty("Material", PROPERTY_FILE, "MAT Files (*.mat):mat", "", "RoadMaterial Files")
Find line 453 or
material=LoadMaterial("abstract::"..self.model:GetKey("material"))
And change the material to Material
I have not tested this extensively, but it should allow you to change the road material easily.
If some one wants to confirm, or show I better solution.