Some screenshots from the track editor:

Built using a custom UI framework for C++ (which is still far from finished).

Road segments are basic cubic splines with direction shared across points. The track has 3 "lanes" - road and left/right sides. Each lane has a specific shape and texture settings.

Fences are the vertical parts of the track. Normally each track has 3 on each side - the actual visual fence near the road, the "forest" and an extra invisible fence for collisions that is higher than the normal one.

Due to the limitations of OBJ files, a separate binary format was created that contains the track meshes and two precomputed traffic lanes. This format is read by a scripted importer in Unity, but could also be used outside of it.