With this package you can create your own race track very easily.
This package comes in your world with 16 track sections, and 20 detail objects. Everything is setup as a prefab object with custom collision models and custom shadow maps. The models are also short, allowing mobile developers to use this package. I have only used standard Unity materials so that it can be tested quickly on any platform.
Note, to snap segments of tracks together, I put V down and snap using the vertex snap tool. Watch the video of how I did it.