  In this paper we introduce the circuit diameter of polyhedra, which is always
bounded from above by the combinatorial diameter. We consider dual
transportation polyhedra defined on general bipartite graphs. For complete
$M{\times}N$ bipartite graphs the Hirsch bound $(M{-}1)(N{-}1)$ on the
combinatorial diameter is a known tight bound (Balinski, 1984). For the circuit
diameter we show the much stronger bound $M{+}N{-}2$ for all dual
transportation polyhedra defined on arbitrary bipartite graphs with $M{+}N$
nodes.
