def _get_bounding_box(self):
        """
        By using this bounding box, the edge length of x, y direction and the center of this road network can be
        easily calculated.
        :return: minimum x value, maximum x value, minimum y value, maximum y value
        """
        boxes = []
        for _from, to_dict in self.graph.items():
            for _to, lanes in to_dict.items():
                if len(lanes) == 0:
                    continue
                boxes.append(get_lanes_bounding_box(lanes))
        res_x_max, res_x_min, res_y_max, res_y_min = get_boxes_bounding_box(boxes)
        return res_x_min, res_x_max, res_y_min, res_y_max

        By using this bounding box, the edge length of x, y direction and the center of this road network can be
        easily calculated.
        :return: minimum x value, maximum x value, minimum y value, maximum y value
        
metadrive/component/road_network/node_road_network.py