I ignored your Rhino file (sorry, unintended) so ignored scale. I started by manually drawing a control point curve of a figure 8:
I set the curve seam to the center point (self-intersection point), divide the curve and moved each point up in Z, passing through a Graph Mapper to ease the slope at the ends. Then used pFrames to create section curves for the roadway.
ramp_2023_Aug16a.gh (19.2 KB)