Roadway Designer - What you see vs. what you get

InRoads does not handle interior boundaries when multiple surfaces are created from separate corridors. The only thing one can do it to manually delete those triangles after the fact. IF you then use the flood fill tool to create a region you can import an interior boundary.
You might need to create it in a 2D file and drape it on the surface and if you do, I would re-triangulate the surface to restore these triangles so the drape command does not result in any gaps.

Okay, I did create this as a single surface from multiple corridors using target aliasing and clipping. It still doesn't do interior boundaries in these conditions? So you are suggesting creating a shape with the borders of the fills and cuts, drape that to the (existing?) surface and then import into the proposed dtm as an interior boundary? What surface am I re-triangulating?

Thanks for the input!

OK, to create a clean outline of the needed "hole" in the DTM, you need to have something as a guide. It is possible to manually draw the outline, snapping to each triangle vertex. But the Create Region by Flood is much faster. It might not work on 3D triangles however, so I mentioned a 2D model for that step.
After the region is created, if it is not fully 3D, it must be draped onto a surface to be used as an interior boundary. An Exterior can be set to Do Not Triangulate, be at any elevation and still function. But an interior boundary cannot.
My thinking was to drape the region onto the proposed surface, but you just made a hole in it. So the draped shape might fall off an edge in the process. By re-triangulating the proposed DTM, the hole will be gone and the shape will drape properly. I recommend this step be completed back in a 3D model.
Once imported into the proposed surface as an interior boundary, retriangulating the proposed surface will recreate the hole.
Any time you delete or swap triangle edges, you cannot retriangulate or those edits will be lost.

OK, to create a clean outline of the needed "hole" in the DTM, you need to have something as a guide. It is possible to manually draw the outline, snapping to each triangle vertex. But the Create Region by Flood is much faster. It might not work on 3D triangles however, so I mentioned a 2D model for that step.
After the region is created, if it is not fully 3D, it must be draped onto a surface to be used as an interior boundary. An Exterior can be set to Do Not Triangulate, be at any elevation and still function. But an interior boundary cannot.
My thinking was to drape the region onto the proposed surface, but you just made a hole in it. So the draped shape might fall off an edge in the process. By re-triangulating the proposed DTM, the hole will be gone and the shape will drape properly. I recommend this step be completed back in a 3D model.
Once imported into the proposed surface as an interior boundary, retriangulating the proposed surface will recreate the hole.
Any time you delete or swap triangle edges, you cannot retriangulate or those edits will be lost.