Vital triangle: A new concept to evaluate urban vitality

Urban vitality is a driving force of urban sustainable development and influences the well-being of its residents. To better understand and evaluate it, we defined urban vitality from the perspective of the urban organism and proposed the "vital triangle" conceptual framework linked to the three aspects of growth, diversity, and mobility. We designed an evaluation index system that includes the six dimensions of population, economy, society, government, environment, and external links by integrating multiple sets of heterogeneous data, and applied it to cities in the Yangtze River Delta. The results show that Shanghai, Nanjing, Suzhou, and Hangzhou have the highest vitality levels but are weak regarding their eco-environments. Cities with high vitality are distributed on the Z-shaped key development belt of the Yangtze River Delta urban agglomeration planning. Regional differences in external links and population vitality are significant, and these two dimensions pose weaknesses to many cities. The interregional differences linked to social vitality are the smallest. Vitality not only comes from within the urban system, but also greatly depends on the external environment. The concept "vital triangle" and relevant evaluation framework are intended to serve as a basis for future research, and the case study could provide some key decision-making support for urban planning and management.