This is a paid asset, but now you can download Race Track Construction Kit Free.
Detail this asset from Unity Store: Original Link
Race Track Construction Kit v4.0
A set of high quality modular race track pieces and furniture, enabling you to construct your own race course in minutes.
Each track piece is 20m square and fully tileable, allowing them to be used in any combination. Most textures are 1024 wide (2 lower resolutions sets are also included), and every material has a colour, specular and normal map.
Everything shown in the screenshots is included, including 2 example tracks.
New to version 4.0: 12 new race track pieces so you can add gravel and concrete to your track edges.
Email: [email protected]
Also available for download, the Race Track Construction Kit: Add-on Pack A and Race Track Construction Kit: Add-on Pack B, two large sets of art assets to add additional details such as crowd grandstands, tyre walls and advertising hoardings to you race track.