Here is a timelapse he gave me of him making a map in Blender
https://www.youtube.com/watch?v=iJIAHungTOchere is the rest of his devblog
https://www.youtube.com/playlist?list=PLk8trZTSfuchEjQfqpaGx0gXMneYsNSiF"basically - you do it very carefully - have him make a grid of triangles, that'll be his base working terrain. you copy and paste that thing, and then sculpt it, the roads are long arrays that you apply to a curve"
also lol

"this is my terrain system in theory:
i.imgur.com/6Za315z.pngblack is road, green is the side of the road,orange is the terrain.
black and green are different model, but follow the same curve. they overlap into each other, and the green curves down at the edges to allow for an overlap onto the rest of the terrain
the rest of the terrain are the triangle pieces you sculpt, and try to fit into the green run-off area"
he wanted you to have this as well, it's an example
https://www.dropbox.com/s/246dfctcklgdy81/SummitHill3BackupForFilipe.blend?dl=0"best is to keep the triangle pieces' position at the center, and move the thing only in edit mode, this makes it easier to line up everything in Unity.
the nurbs path can be extruded, and the array modifier can have more array pieces to make the road longer.
Roads should have relatively small segments, as shown in my own road model, to allow for smooth driving. I have a loop cut about every half meter"