Hey, this problem with LOD´s is known, is still not work.
LOD´s for Roads make you self.
Circle with 32 segments, as LOD Object create Circle with 8 segments.
Work best always with Low Poly, this is better as more poly (triangle)
More Triangle = More Work for Graphics Card and result lower Framerate
You must all textur with lower resolution, standard is 1024x1024 lower is 512x512 or 256x256
LOD-Objekt and Textur Name
Name_LOD.fbx for LOD-Object
for example:
Wall.fbx for you Full-Resolution Wall-Object
Wall_lod.fbx for you Low Resolution of Wall-Object
Textur:
Ziegel_d.png or Ziegel_d.jpg for Full Resolution of Textur Diffius
Ziegel_lod_d.png or Ziegel_lod_d.jpg for low Resolution.