The characteristics of traffic flows on signalized arterials are examined within a cellular automaton microsimulation model. The model is used to analyze arterial throughput and travel times for given densities, coordination schemes, and signal spacings. A fundamental three-dimensional relationship is established between flow, density, and offsets for signalized arterials. In particular, it is shown that arterial throughput is dependent on offsets and that the constituent single-intersection limiting capacity, as determined by the saturation flow and the green splits, can be realized only under optimal coordination conditions for a limited range of densities on the arterial. This finding is a manifestation of the important role that signal coordination and, in fact, intelligent transportation systems in general play in the operation of urban street networks.