Creat another surface "Void-1". Paste the non-roadway surface into it. Add a boundary that matches the void area inside the roadway. Paste the three surfaces into your combined surface in this order: "Non-roadway surface", "roadway surface", "Void-1 surface".
this is the approach I'd take although just check that the triangles form correctly at all common boundaries. They usually do but occasionally you might have a gap which can be closed by adding a show boundary.
neilyj (No connection with Autodesk other than using the products in the real world)
IDSP Premium 2015 (mainly Civil 3D 2015 UKIE & Infraworks with some limited 3ds Max Design) Win 7 Pro x64, 500Gb SSD, 1Tb HDD 32Gb Ram Intel Xeon CPU E5-2650 v2 @ 2.6GHz (32 CPUs), ~2.6GHz NVIDIA Quadro K4000, Dual 27" Monitor, Dell T5610