Please see our project done in gaea and vray. Lots of usefull informaiton on here that helped is get to this result. There where a few questions about how to add roads in gaea. We ended up creating a basical primative terrain in maya with the roads and rendering a hight map, we then rendered a road only highmap and in gaea used the road only hight maps to generate a mask and used this mask in gaea to mask out the errosion and various other nodes around the road.
Full project can be seen here https://additivestudios.co.uk/alpine-showroom-image