Most new track makers, including myself, have had an issue where the road ends up being bumpy on banked corners in STK, even when the curve is smooth in the editor. (The first image doesn't look too bad but you can really notice it when you drive.)
The reason for this is because the rectangles ('quads') are automatically turned into triangles when exporting. These big triangles end up making the road angular and bumpy to drive on.
To smoothen the road, subdivide the road surface so that instead of being one big rectangle, it's many smaller rectangles. If your road is 6 units wide, I suggest setting 'number of cuts' to 5 or more (there are many tutorials on subdividing, it's quick and simple). The more cuts, the smoother the road is, but also more triangles it creates which make it take more computing power to render, so it's a waste to make too many cuts (4-6 should be smooth enough for perfect driving, 10 cuts is probably a waste).
Here I've only made 2 cuts, but you can still see the massive difference.
I hope this hint gets put in the official documentation or any future tutorials since it's a common issue.
Also, remember to upload your track source Blender files when you create an add-on!