Figured it out by my self.
Thanks.If you do it that way you can use the metal detail texture to add close-up details (usually metal flake but carbon fibre and other stuff's also done)
1/ Do you want to intersect your current spline with the outer line of the blue (road?) shape ?How to connect vertices to then create polys (with Editable Poly modifier)? So far this is now a "in-all-directions" curved spline. If I try to use a Subdivide modifier it does not work correctly, since it only adds flat surfaces, and some huge step in between because of the elevation. Surface modifier doesn`t work either.
I would have to connect bottom and top vertexes, so I can get a curved surface. But how?