Balancing Accessibility and Equity in EMS Resource Allocation: A Spatial Optimization Perspective

Background Emergency Medical Services (EMS) play a crucial role within the broader field of emergency medicine. Efficient resource allocation is vital to ensure timely access to care and equitable service distribution. However, when resources are allocated inefficiently, service areas can become either excessively large or overlap. Oversized service radii can delay response times, while overly small radii result in the inefficient concentration of resources, ultimately compromising service equity. Thus, optimizing the allocation of EMS resources is essential to improve both response times and fairness in service delivery. Methods This study integrates the spatial characteristics of supply and demand in EMS systems and employs accessibility-based measures to optimize resource allocation, aiming to improve both accessibility and equity. A Supply-Demand Three-Step Floating Catchment Area (SD3SFCA) model is developed to evaluate service levels. Based on this, a static bi-objective optimization model is constructed-balancing accessibility and equity-and solved using the NSGA-II algorithm. Model performance is examined under varying impedance coefficients to assess robustness and effectiveness. Results The results reveal that accessibility improves with increased investment in emergency facilities, eventually reaching a plateau. In contrast, equity follows an inverted U-shaped pattern—rising initially and then declining. Under equal equity levels, lower impedance values correspond to higher accessibility, indicating an inverse relationship between impedance and accessibility. Similarly, under equal accessibility levels, equity improves with lower impedance before congestion effects emerge. Furthermore, lower impedance coefficients yield more Pareto-optimal solutions within acceptable bounds, while higher impedance limits solution diversity and leads to suboptimal outcomes near the edge of feasibility. Conclusions These results underscore the importance of low-impedance transportation environments-achievable through infrastructure upgrades or EMS traffic prioritization-in improving both accessibility and equity. The proposed SD3SFCA-based bi-objective optimization model demonstrates strong robustness across varying urban contexts, offering a theoretically sound and practically applicable framework for the planning and evaluation of pre-hospital emergency medical transport systems.