The airplane was taxiing for takeoff at night on a 1,463 feet long, snow covered, unlit and unmarked, taxiway. The pilot inadvertently taxied onto an unmarked vehicle road which crossed the taxiway at a shallow angle. He realized he was not on the taxiway when he saw vehicle headlights in front of him. He turned the airplane toward the runway lights, and the airplane dropped into a ditch. Since being stationed at the airport, the pilot had used the taxiway three times, and always in the daylight.