Does the resolution of the weightmaps you intend to use suffice for drawing roads? They also won’t be directional mapped. It still isn’t specific enough, do you stuck at the shader code? There are examples, but those i remember face a different issue and aren’t made with THREE.
Just on a side note, i’m releasing a large scale terrain engine next week, it uses a method to support up to 256 materials at once including normal and displacment maps, fast with culling and LOD. It’s purpose is to enable large scale static-data terrains, also aiming to provide a road solution, since it isn’t trivial to solve. It’s a component of a larger framework of mine.