Thanks Allen and Miguel.  I appreciate the responses!     Miguel, conceptually, I think I understand what you are suggesting.   In terms of a work flow or process, I am a bit confused.   Do you create the roadway, update the DTM, then trace over the length of the parts using a 3d poly, cut then paste the 3d polys into the existing data of the DTM, update the model,  delete the original roadway?   Thanks,    Jerry