In both of your fooldand text files here are the codes and explanations:
The first 4 objects below can be driven on ( code is zero on the first number ).
The 4th one" (trees-buildings-signs-etc.mod, 3, 0, 0, 0)" has a 3 in front of 3 zeros.
This 3 code tells the game that it is not driveable or hittable and can sit on top of a surface that is driveable.You cannot put a driveable object on top of another driveable surface or the game creates an invisible hole.
Overlapping polygons also creates a hole.
Only this code "3, 0, 0, 0" as shown below is acceptable.

  modobject(pavement1track.mod, 0, 0, 0, 0)
  modobject(sticky-grass.mod, 0, 0, 10, 0)
  modobject(gravel1road.mod, 0, 0, 16, 0)
  modobject(any-water.mod, 0, 0, 14, 0)
  modobject(trees-buildings-signs-etc.mod, 3, 0, 0, 0)