Generating a 3D roadmap out of a 2D roadmap

To state my problem shortly I have to make a 3D roadmap out of a 2D roadmap with the only information about all connecting 2D road segments and bridge locations. As a consequence I have to setup the heigths and gradients of each segment of the entire road map without breaking some constraints (for example intersecting driveways). As in my opinion a procedual generation won't work so I came up with the idea to integrate a solver engine, but I'm not sure if this is the optimal solution. Have maybe someone a solution for that problem? Or have someone already worked with the integration of a solver engine into unity and could recommend a unity compatible solver engine, too?