  We give sufficient conditions on the rates of two asymmetric exclusion
processes such that the existence of a blocking invariant measure for the first
implies the existence of such a measure for the second. The main tool is a
coupling between the two processes under which the first dominates the second
in an appropriate sense. In an appendix we construct a class of processes for
which the existence of a blocking measure can be proven directly; these are
candidates for comparison processes in applications of the main result.
