Morphological dynamics of cost-efficient urban areas: Expansion and shrinkage in greater Tokyo from 2003 to 2020

The delineation of cost-efficient urban areas, considering their morphological dynamics including urban expansion and shrinkage, is essential to compare present and future urban areas and evaluate the effectiveness of policies to achieve a cost-efficient and spatially compact city. Nevertheless, we lack the methods for investigating the relationship between the dynamics of cost-efficient urban areas and the associated costs of infrastructure connections (road networks). To address this issue, we develop a new method for investigating this relationship. The results of a case study conducted in the prefectures surrounding Tokyo showed that (1) most cost-efficient urban areas are statistically categorised as being at equilibrium, indicating no significant urban expansion for new building generation, aggregation, or urban shrinkage regarding vacant plot generation and fragmentation; and (2) the minimum connection cost per building ranges from 3.8 to 7.7 and tends to be temporally stable in each prefecture. Therefore, we conclude that cost-efficient urban areas are stable in their dynamics and connection costs per building. The stability regarding morphological dynamics, the optimal criteria for determining the spatial extent of cost-efficient urban areas and the minimum cost per building are significant when we compare present urban areas with cost-efficient ones.