Residential density and 20-minute neighbourhoods: A multi-neighbourhood destination location optimisation approach

The concept of 20-min neighbourhoods, a planning intervention to promote local living and active travel, has recently become a key urban planning priority in Melbourne, Australia. The Victorian Government defines this concept as being able to reach daily local living destinations within a 20-min round-trip walk from home, which is approximately 800m of walking for each trip leg. Similar concepts, such as 10- or 15-min cities, have been explored in other cities worldwide. However, research to date has largely focused on measuring accessibility to destinations in developed areas, with few studies examining what is needed to build a city of 20-min neighbourhoods, i.e., investigating multiple neighbourhoods, shared destinations, and the role of residential density. In this study, we used optimisation models to examine the relationship between residential density and the targets of 20-min neighbourhoods in a hypothetical greenfield development scenario. We defined different targets for various destinations in terms of the percentage of the population aimed to have access to that destination within 800 m, ranging from 95% for the smallest destinations to 70% for the largest. Our results demonstrate that at least 25 dwellings per hectare (assuming 2.6 persons per dwelling) are needed to provide access to the destinations within 1.2 km and 35 dwellings per hectare to provide access within 1 km. Furthermore, we show that the cost of building destinations and the land required for them when delivering 20-min neighbourhoods at 30 dwellings per hectare is almost half of what is required when building at 15 dwellings per hectare.