Studies by Assistant Professor Anne Goodchild of the University of Washington and Professor Carlos Daganzo of the University of California-Berkeley show such double cycling of cranes would also save costs and conserve fuel.

Typically, a quay crane unloads all the containers from a ship. After dropping the container on the dock, the empty crane swings back to pick up another load. It continues to do this until the ship is empty and then starts to load outgoing containers onto the ship.

Double cycling seeks to maximize crane operation by unloading one container and then immediately picking up a new container on the dock to load onto the ship. But the technique is used at less than a quarter of terminals, Goodchild estimates, because of fears it would complicate port operations.

"When terminals do use double cycling they only do it below deck, and then only within one bay," Goodchild said. "I would suggest that be implemented much more broadly."

The study is to appear in the journal Transportation Research B.