Instead of using the terrain or spectator seat mesh directly, you should create a separate mesh for the spectators.
Open Track in Blender (or any 3D software),
and use the Knife tool, Loop Cut, and Vertex Slide to edit and separate the mesh.
Make sure that it does not overlap with walls or other objects.
View attachment 1467688
In the case of spectator seats, reduce the mesh slightly so that the spectators do not protrude from their seats.
View attachment 1467691
Simply reducing the ELEMENTS value will not solve the problem fundamentally.