Examples of Terrain Alignment for Unity. I put it on github for educational purposes in case someone wants to toy around.

It works by projection of gameobjects and then blending that projection with the heightmap. Currently the projection works with a camera that's the full terrain size. Of course especially for performance reasons one would only project the gameobject itself or only delta changes in case of a spline road or a brush painted path, but that'll come at a later state while this tool evolves.

Important: This is only experimental so far, install into a new project or backup your terrain!

Random Mesh

Road Mesh