...
- In max, duplicate all road meshes and merge them.
- Remove all UVW data, and then acquire a UVW map modifier in channel 1 with the same mapping as the RoadDetailMap uses.
- Before you start vertex painting, you need to set all the vertices on the model to black (make sure you unhide all vertices as well to make sure you don't miss any).
- Paint in Vertex Colour the areas you wish for dust, groove and wear to appear.
- Use separate Red, Green and Blue vertex colour modifiers with add blending mode on the 2nd and 3rd ones.
- Paint using brush with around 5m width
- No need to be super accurate, this is an overall mask. We will add detail in substance.
- Clone the multi material and convert to standard materials.
- Export an FBX file of the object.
...