Functional Optimism Bias in EMS planning: Structural distortions of accessibility in high-density megacities

Current evaluations of Emergency Medical Services (EMS) accessibility rely on Traffic Analysis Zones (TAZ), yet the bias introduced by this unit remains poorly quantified. Using Shanghai as a case study, this study measures EMS accessibility across eight spatial scales, from 1 km grids to administrative districts, based on network travel times derived from 9314 origin–destination (OD) pairs. The results show that TAZ-based estimates overstate service coverage by 21.5% relative to fine-grained assessments and follow a non-monotonic inverted-U pattern across scales, with the largest bias at intermediate levels. This overestimation arises from centroid-based representations that concentrate trips on arterial road networks. At the same time, intra-block travel delays within large gated superblocks are not captured, leading to the systematic misclassification of underserved neighborhoods. These findings demonstrate that zone-based methods can mask localized accessibility deficits and highlight the importance of fine-resolution, origin-based measures for accurately identifying service gaps in high-density megacities.