  We explore driven lattice gases for the existence of an intensive
thermodynamic variable which could determine "equilibration" between two
nonequilibrium steady-state systems kept in weak contact. In simulations, we
find that these systems satisfy surprisingly simple thermodynamic laws, such as
the zero-th law and the fluctuation-response relation between the
particle-number fluctuation and the corresponding susceptibility remarkably
well. However at higher densities, small but observable deviations from these
laws occur due to nontrivial contact dynamics and the presence of long-range
spatial correlations.
