using the highway interchange geometry from my 2d tutorial i created profiles and splines to sweep the forms of the roads.
in order to keep the different types of surfaces constrained, i assigned real parameters for the width, depth, and offset from the ground.
to keep the loop connectors attached to the straight runs i created a real parameter that set the length between the sweep spline and object to 0 (or any other assigned value)
finally, i flexed the real parameters to transform almost flat planes into volumes. setting widths equal to depths, the highways took on an oddly spatial quality.