Working with splines

Hey all,

working on a tool for automated placement of splines/meshes for road systems.

I have an issue with the street meshes. The roll is wrong, and part of the meshes sinks below the ground.

I have seen that the spline tangents dip below the landscape, and manipulating the tangents by hand could potentially be a fix, though this does not fix the issue with the roll.

Any pointers are greatly appreciated, particularly if I can implement them with C++.