I think autocross is just small enough to split it up in LOD parts, but not big enough to really make it forget parts you are far away from. Wich a city, would be...
About the path, the parts the sim forgets, are being generated again using a certain path, so it only renders the road near you. So it knows when to load what parts, because you are following the path it knows. In a city you can't set a path, so the engine wouldn't know what part to load up next.