good job here, but I'm going to be critical. it looks good, but there still a lot of work to do. all these 3d objects that are being used under "wall" for collision data need separate meshes, invisible meshes and lower poly to manage all collision data. just like the physical road being invisible, all collision data needs handled that way and as low poly as possible. even the house the car would never touch is titled as physical wall.
Ks grass should be used for 3d upright grass, not the flat terrain grass.
far to much 3d grass in grass kn5. at its thickest it should be about 0.5m - 0.7m between grass pieces. half that volume if you can.
road layout is quite good, however the visual version can be even lower polygon if you want to save more.
many people having issues with the track, including me, it crashes my AC. but I have to say its lovely looking at first glances, and then its too much for some peoples pc`s.
. It looks like you have put a lot of effort into this track. Take the comments from the guys as constructive criticism and maybe it will become even better. I can't test it right now, but I hear some people are having problems with performance. You can take a look at these tips HERE, at least to get an idea of how AC "sees" the tracks.