Sustainability of cities under declining population and decreasing distance frictions: The case of Japan

This study develops a statistical model that integrates economic agglomeration theory and power-law distributions of city sizes to project future population distribution on 1-km grid cells. We focus on Japan -- a country at the forefront of rapid population decline. Drawing on official population projections and empirical patterns from past urban evolution in response to the development of high-speed rail and highway networks, we examine how ongoing demographic contraction and expected reductions in distance frictions may reshape urban geography. Our analysis suggests that urban economies will consolidate around fewer and larger cities, each of which will experience a flattening of population density as the decentralization of urban populations accelerates, while rural areas are expected to experience further depopulation as a result of these spatial and economic shifts. By identifying sustainable urban cores capable of anchoring regional economies, our model provides a framework for policymakers to manage population decline while maintaining resilience through optimized infrastructure and resource allocation focused on these key urban centers.