made in unity, watch the on-track car view at the end
You appear to have missed the blog he linked to in the description of the video. He describes the techniques he used complete with pictures.
Oh, I missed the lil explanations. :o
but…
“First a track surface mesh is build based on a 3d spline curve”
…how does that even work? 
Also, there is a procedural mesh generation example project in the resources section of the Unity website.
http://forum.unity3d.com/viewtopic.php?t=23519
Apart from trees / buildings, this does the same by manually positioning markers. At the moment only the road is created. Version 2 will allow you to create any kind of procedural geometry and side objects.