Effectiveness of emergency medical services (EMS) depends on a wide range of decisions in its planning and operation phase such as ambulance locations and dispatching protocols. Much research has been conducted on EMS design and operational decision making in order to improve the quality of EMS systems. It is often the case that these research works focus on a decision problem on a specific aspect and tend to overlook possible interactions from other elements of an EMS system. This paper introduces a simulation model as a generic EMS system design evaluator, where a wide range of design and operational factors are comprehensively incorporated. Experiments using the developed model show that there exist interactions among many design and operational factors in an EMS system, which demonstrates the importance of considering all decisions when developing solutions for a specific decision problem in EMS design and operation.