This is a perfect track for high speed testing, it's just an immense perfectly flat surface.
It can allow a maximum of 24 cars, but AI is not working (can be used online or as reference point to find the center of the plane).
There are 300 circles which diameter is different 1km by one (so if you do a drag test, every line is 500m far from the next), that you can use them as reference point or for some drag test.
This isn't really endless: if Assetto Corsa takes the measures well, the track should be 400km wide and long, a perfect square.
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Endless Floor test track 3.0
Huge plane track, perfect for high speed testing