  Proximity-based cities have attracted much attention in recent years. The
15-minute city, in particular, heralded a new vision for cities where essential
services must be easily accessible. Despite its undoubted merit in stimulating
discussion on new organisations of cities, the 15-minute city cannot be
applicable everywhere, and its very definition raises a few concerns. Here, we
tackle the feasibility and practicability of the '15-minute city' model in many
cities worldwide. We provide a worldwide quantification of how close cities are
to the ideal of the 15-minute city. To this end, we measure the accessibility
times to resources and services, and we reveal strong heterogeneity of
accessibility within and across cities, with a significant role played by local
population densities. We provide an online platform
(\href{whatif.sonycsl.it/15mincity}{whatif.sonycsl.it/15mincity}) to access and
visualise accessibility scores for virtually all cities worldwide. The
heterogeneity of accessibility within cities is one of the sources of
inequality. We thus simulate how much a better redistribution of resources and
services could heal inequity by keeping the same resources and services or by
allowing for virtually infinite resources. We highlight pronounced
discrepancies among cities in the minimum number of additional services needed
to comply with the 15-minute city concept. We conclude that the proximity-based
paradigm must be generalised to work on a wide range of local population
densities. Finally, socio-economic and cultural factors should be included to
shift from time-based to value-based cities.
