How to reduce number of triangles in terrain model on openroads?

Marek Demczyna said:

I can not import point cloud again

Actually, if you're working with Point Clouds using a large dataset, you can reduce the density of the POD to as little as 2% and create the terrain from that reduction.  Additionally, you can export that reduced POD file, then remove the original and reimport the reduced POD version allowing you to refine it down further from 100% of the reduced density.  

Marek Demczyna said:

Is there an option to filter triangles after the slope to merge triangles with similar slope?

I'm not aware of such a process in ORD.  

It might be a bit cumbersome, but one way to reduce the amount of triangulation is simply by dropping the terrain model.
Then create a mesh from it (ensure you're in the 3D model, go to the modeling - mesh ribbon), where you can also find functionality to simplify a mesh.
Once the mesh is simplified, you can then create terrain from it again using the terrain functionality in the openroads ribbon.

Thank you for your advice.

I will have to look another solution.