I have a road that has a fairly large cut section but when I take it to Lumen to render and animate it the existing ground shows up on top of the proposed surface.
So the only design surface I can see is areas that the grade is higher.
Create a clipped terrain that removes the existing ground from under the proposed road before sending the model to LumenRT.
Is there a video or workflow available to show how this is done?
One example - old version but workflow is still valid.
https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/17434/video-terrain-model-clipping
Basically, any shape can be used to clip a terrain, either Internally or Externally. You could also create a terrain of the proposed road, and use that to clip the existing terrain.