Autodesk Civil 3D 2007 contains an undocumented command allowing to easily export (publish) the terrain surface, road, etc. to a .KML file for Google Earth.
Assign a coordinate system to your drawing, model your road or other 3D terrain model and create a 3D surface.
On the commandline enter the EXPORTGOOGLEEARTH command (or AeccExportGogleEarth, or EXPORTKML), select the surface and name the resulting KML file.
Then you can load the KML file directly into Google Earth.
More advanced functionality is available in the "Autodesk Civil 3D 2007 Extension for Google Earth" (technology preview) offered by Autodesk for all Civil 3D 2007 users and in the "Google Earth Extension for AutoCAD 2008". It contains an interactive wizard for publishing your project to a KML file for Google Earth, Google Earth Plus and Google Earth Pro.
GE Extension can be found on the Autodesk Labs web site.