The part of the Eq. (5), which is given in the parenthesis, represents a simplified version of the summation of
the average travel distances in all dimensions. The carrier starts the carrying process of an item at the dock. It
carries the item to the shelf and return to the dock. It means that the distance is passed two times. At the end
of the storage time, the same item is picked back to the dock and this means two more passing the same distance.
This situation is represented by adding a leading coefficient 4 in Eq. (5).