*Some of the tree trunks were above the car when they should be below the car. Move the tress further away from the track if you dont want to use multiple layers of trees.

*you should not be able to turn when car is stopped. At least turning should be slower.

*why exactly is the grass making the car bump back? I would understand if the track would have some sort of walls around it, but if you drive on the grass it should only slow you down.