Faced with a mismatch between demand and resources, Emergency Department (ED) administrators and staff need to gauge the impacts of staff decision processes in lieu of increasing resource levels. In this paper we present REDSim, a spatial agent-based simulation framework for studying emergency departments. REDSim focuses on quantifying the impacts of staff decision processes, such as patient selection, on the length of stay, waiting and boarding times, and other variables. We show REDSim's effectiveness by comparing four patient selection strategies: longest waiting, shortest distance, random, and highest acuity. REDSim showed that although patient length of stay is not significantly reduced (1.4%) the throughput increases 17% when providers select the closest instead of the highest acuity patient for the next task.