This paper addresses the throughout improvement possible using a dual-
shuttle automated storage/retrieval system. Travel-between time in a d
ual-command cycle is virtually eliminated, resulting in large throughp
ut improvement. The system is extended to perform an equivalent of two
dual commands in one cycle in a quadruple-command mode (QC). A heuris
tic that sequences retrievals to minimize travel time in the QC mode i
s developed. Monte Carlo simulation results show that the heuristic pe
rforms well, achieving large throughput improvement compared with that
of the dual-command cycle operating under the nearest-neighbor retrie
val-sequencing heuristic.