15-minute city beyond the urban core: Lessons from the urban-suburban disparity in PCR accessibility within the X-minute framework

• This study seeks to move 15-minute city studies beyond the urban core. • It evaluates urban-suburban disparities in PCR accessibility in 10 megacities of China. • Shapley decomposition reveals the distinct impact of PT schedules and testing site hours. • Recommendations: extend testing site hours and improve PT systems in suburban areas. • Contribute to creating more resilient, sustainable, and inclusive urban environments. The 15-minute city concept has garnered increasing attention as a transformative urban planning paradigm to enhance accessibility, sustainability, and livability. However, critical gaps remain in its practical application. Current studies predominantly emphasize urban cores and active transportation modes while neglecting the nuanced challenges of suburban areas and the pivotal role of public transit (PT). This study critically examines the 15-minute city framework through the lens of accessibility disparities, using Polymerase Chain Reaction (PCR) testing facilities across 10 Chinese megacities as a case study. Our findings highlight that suburban residents face significantly greater accessibility challenges, including longer travel times and reduced access during nighttime, compared to their urban counterparts. The results underscore the limitations of a narrow focus on active modes in the urban core within the 15-minute city framework, revealing that PT schedules and facility operating hours distinctly affect accessibility outcomes in suburban neighborhoods. Solely focusing on 15-minute accessibility will underestimate their impacts. We advocate for an expanded framework that integrates a 15-minute city model for urban cores with a 15-30-45-minute approach for suburban areas, leveraging PT to address diverse transportation needs. Our findings advance theoretical and methodological approaches to the 15-minute city, offering actionable insights for policymakers to develop more inclusive, adaptable, and equitable urban planning strategies.