As far as I remember, Marius_A is currently using a custom list of waypoints that he obtained by "flying" the desired route with the ufo and applying a corresponding terrain/altitude offset.
So it does not exactly "scale" right now - but could be easily augmented via data from other efforts, e.g. OSM - as long as we have vector data of road networks, we could try to reproduce such vector data inside in fgfs and use that to drive the corresponding back-end logic and animate moving traffic
Another possibility would be using a moving-map style display in conjunction with some road networks and the route manager to create such traffic procedurally.
For instance, the way Stuart has been structuring the FG1000 moving-map device, it would be relatively straighforward to hook up the same device to a different (non-aircraft) back-end, which includes boats/vessels, cars and other vehicles:
http://wiki.flightgear.org/FG1000