Measuring the dynamic accessibility to COVID-19 testing sites in the 15-min city: A focus on service congestion and mobility difference

Accessibility to essential amenities such as food and health is crucial for all individuals and is the key to realizing the goal of the 15-min city. However, so far, few studies have fully considered the service congestion of the facility and mobility difference among age groups in accessibility modeling. Here we propose the improved Three-step Floating Catchment Area (3SFCA) method that incorporates the changing congestion degrees to capture the dynamics of the facility and assigns differentiated catchment areas to children, young adults, and older adults according to their differences of travel speeds by walking and cycling. We apply this approach to investigate the spatiotemporal pattern of accessibility to COVID-19 testing sites in Shenzhen across the day and evaluate the horizontal equity and vertical equity by using the Lorenz curve and Spearman's rank correlation coefficient. Our results suggest that the allocation of testing sites aimed at the 15-min city is more in line with the 20-min city in reality. The accessibility distributes dispersedly across the city and varies more dramatically in western and eastern peripheral areas. In terms of equity, the whole accessibility distribution is more unequal after considering children and the elderly's limited use of cycling and reduced travel distance. The vertical index also reveals that the Luohu district should give more priorities to these vulnerable people. The findings may assist city planners and policymakers in improving the allocation and management of primary resources in the 15-min city and implementing more effective measures to prepare for the uncertain future.