It is known that the map has terrain problems...potholes cause car damage and very rarely cars can sink/fall through the ground.
Probably won't be fixing the potholes as it helps add realism.
Not sure how to prevent the cars from falling, but it's very rare, so if that happens, just get out of the car and fall to the bottom and drown. Dying is part of the game anyway, just consider it to be another way to die in this deadly sport!