This paper analyzes the traffic flow stability/instability induced by arbitrary Adaptive Cruise Control spacing policies. It is shown using a microscopic model, that for any monotonically increasing spacing policy, the induced traffic flow on a circular highway is asymptotically stable. When a spatially discrete macroscopic highway model is considered, the traffic will be stable when the biasing strategies or mixing parameters are chosen appropriately, depending on the slope of the density-flow curve. This result indicates that when macroscopic highway model is considered, consistent biasing strategy must be chosen to be consistent with the microscopic model. These results therefore extend our previous results for the Constant Time Headway (CTH) policy to generic ACC policies. Based on these results, a complete analysis of the effect of boundary conditions on traffic stability for open stretch highways populated by ACC vehicles is performed. These results have been verified by microscopic simulations.