Mapping Public Transport Accessibility for Urban Health: A Spatial Method Applied to Milan

Abstract Background Public transport accessibility is a key social determinant of health, influencing access to healthcare, education, employment, and social inclusion. A standardised metric to assess accessibility is lacking. This study introduces a data-driven methodology to evaluate public transport accessibility at high spatial resolution and support equitable public health planning. Methods A two-grid spatial model was applied to the province of Milan, with a fine grid (500m x 500m) to generate localised accessibility scores, and a coarser, clustered grid (50 points per province) to optimise travel-time calculations. Only points within 500 meters of a transit stop were included. Travel times were retrieved using the Google Maps API. Scores were calculated as average speed (air distance divided by travel time), then normalised. To assess the score's relevance at municipal level, correlation tests were conducted with the national Index of Accessibility to Essential Services (car travel to hubs with hospitals, schools, and train stations), the Composite Fragility Index, and road crash incidence. Results Central areas of Milan showed significantly higher accessibility than peripheral zones. Preliminary findings align with known infrastructure and service patterns, supporting the metric's robustness. The two-grid and clustering approach reduced computational burden while maintaining analytical precision. At municipal level, the score correlated with the Index of Accessibility to Essential Services (r=-0.24, p = 0.005), the Composite Fragility Index (r = 0.37, p < 0.001), and road crash incidence (r = 0.23, p = 0.009), reflecting greater transport density in well-connected areas. Conclusions This methodology offers a scalable public health tool to identify underserved areas and guide equitable transit planning. It highlights transport access gaps and associated urban health risks. The scoring system is supported by open-source code, enabling reproducibility and broader adoption. Key messages • Public transport accessibility directly impacts urban health outcomes and service access. • The methodology offers a scalable tool for identifying transport-related health risks in cities.