Intl. Trans. in Op. Res. 21 (2014) 581–617 DOI: 10.1111/itor.12083 INTERNATIONAL TRANSACTIONS IN OPERATIONAL RESEARCH Recommendations for dispatching emergency vehicles under multitiered response via simulation Kanchala Sudtachat a , Maria E. Mayorga b and Laura A. McLay c a Department of Industrial Engineering, Clemson University, 110 Freeman Hall, Clemson, SC 29634, USA b Department of Industrial and Systems Engineering, North Carolina State University, Campus Box 7906, Raleigh, NC, USA c Department of Industrial and Systems Engineering, University of Wisconsin-Madison, Madison, WI, USA E-mail: kanchas@g.clemson.edu [Sudtachat]; memayorg@ncsu.edu [Mayorga]; lmclay@wisc.edu [McLay] Received 30 December 2013; accepted 9 February 2014 Abstract Emergency medical service (EMS) systems provide medical care and transportation. While many real-world systems use multiple vehicle types to attend different call priorities, few guidelines exist about which vehicles to allocate in multitiered responses where more than one vehicle is sent per call. This paper makes recommenda- tions for multiple-unit dispatch to multiple call priorities based on simulation optimization and heuristics. The objective is to maximize the overall expected survival probability of patients classified as “life-threatening”. We assume two types of medical units and three call priorities; and that information may be updated when the medical unit arrives on-scene. First, we study the optimal dispatching policies through several examples. Numerical results show that dispatching while considering call priorities, rather than dispatching the closest units, improves EMS system effectiveness. A heuristic algorithm is developed for large-scale problems. A comparison between the heuristic and closest policy is demonstrated using real-world data. Keywords: emergency medical service; simulation; multiple-unit dispatch; call priorities 1. Introduction Emergency medical service (EMS) systems are operated with the underlying goal of maximizing survival probability of patients. However, most EMS systems use measures of efficiency to evaluate their performance, such as average response time and expected coverage, which could in turn affect patient survivability. Coverage refers to the proportion of patients who can be attended by ambulances within a predetermined time or distance. Response time refers to the time from when an ambulance dispatches to when an ambulance arrives on scene (see Fig. 1). Most operations decisions involved in EMS systems affect response time, such as ambulance location, ambulance relocation, and ambulance dispatching. As the main focus of this paper, we consider the ambulance C  2014 The Authors. International Transactions in Operational Research C  2014 International Federation of Operational Research Societies Published by John Wiley & Sons Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main St, Malden, MA02148, USA.