The FREE version of EasyRoads3D, the easy way to create roads in Unity and have the unity terrain conform accordingly.
For the full toolset and demo videos check
EasyRoads3D Pro which is also available on the Asset Store and still temporarily only $45.
The package includes a simple demo scene in Edit Mode so roads can be changed and added before adjusting the terrain to the roads.
The manual is located in the EasyRoads3D root directory and includes a Quick Start section.
The Introduction Video on this page is made using EasyRoads3D Pro. The package setup, import and road specific options is the same in the free version.
Forum thread:
EasyRoads3D v3 Website:
http://www.easyroads3d.com