You have too many tracks'control points on turns/or their "arms" are too stretched out (image 1) and road's polygons overlap on each other.
Delete some control points, or manipulate with the arms - make them shorter (image 2)
If the road is too rough and not smooth - add some shape (or material) cross-sections and make panel length shorter at the places where you want. (image 3) The shorter panel length you set, the more polygons will be created, so I suggest you find the best setting just to make the road nice. For my task 2,5 meters was enough.
I'm afraid you will have to delete some terrain before you do it, because the terrain's polygons will look bad when you touch roads' nodes. You can just delete some track anchors and it will delete the first row of terrains' polygons. Proceed with caution because btb can crash if you move the "track anchors". Just click them gently and press delete. If you click them in 2D View, they first will merge, because there are always two of them (unless you uncheck "Anchor Auto Merge") (image 4)
After you've done, add missing terrain's polygons using "Click and Drag" technigue and merge according terrain's vertices. You can also merge Terrain Areas afterwards.