With this package you can create your own race tracks very easily.
This package comes with 16 track sections, and 20 detail objects to place in your world. Everything has been setup as prefab objects with custom collision models and custom shadow maps. The models are also lower, allowing mobile developers to use this package. I only used standard Unity materials so it can be tested quickly on any platform.
Note, to snap the tracks sections to together, I held down V and snapped using the vertex snap tool. Check out the video to see how I did it.