Abstract
The Emergency Medical Service (EMS) Act specifies the fraction of all demands for service that must be reached in a given amount of time for urban and rural areas. The conditions have traditionally been interpreted to apply to the case in which all EMS vehicles are available to respond to demands. A model that considers the probability of a vehicle being busy is formulated and model properties are briefly discussed. The model is then applied to two problems:a 55-node test case and a 33-node census tract representation of Austin Texas. The implications of the new model for EMS system design are discussed as are the limitations of the modelling approach.