Enhanced efforts in the transportation sector should be implemented to mitigate the adverse effects of CO₂ emissions resulting from zoning-based planning paradigms. The concept of a 15-minute city, emphasizing proximity-based planning, holds promise in reducing unnecessary travel and progressing towards carbon neutrality. However, a critical research question remains inadequately explored: to what extent is the 15-minute city concept feasible for American cities? This paper presents a comprehensive framework to evaluate the 15-minute city concept using SafeGraph Point of Interest (POI) check-in data across 12 major American cities. Our findings suggest a prevailing reliance on cars among residents due to the spatial distribution of essential activities beyond convenient walking, cycling, and public transit distances. Nevertheless, there exists significant promise for realizing the 15-minute city vision, given that most residents' daily activities can be accommodated within a 15-minute radius by low-emission modes transportation modes. When comparing cities, it appears that achieving a 15-minute walking city is more feasible for metropolises like New York City, San Francisco, Boston, and Chicago, while proving to be challenging for cities such as Atlanta, Dallas, Houston, and Phoenix. In examing inter-group comparisons, neighborhoods with higher proportion of White residents and higher median incomes tend to have more accessible POIs, with a substantial percentage of activities concentrated within a 15-minute radius. This demographic also shows a greater propensity to fulfill daily activities through walking, cycling, or public transit trips within a 15-minute travel time, thus presenting a greater potential in CO₂ reduction compared to African Americans. This study can offer policymakers insight into how far American cities are away from the 15-minute city concept. It also highlights the potential CO₂ emissions reductions that could be achieved through successful implementation.

This paper discusses possible long-term effects of COVID-19 on activity-travel behaviour. Making use of theories and concepts from economics, psychology, sociology, and geography, this work argues that lasting effects can be expected, and specifically that peak demand among car and public transport users may be lower than if the pandemic would never have happened. The magnitude of such effects at the aggregate level in terms of the total travel time of all inhabitants of a country or state is likely limited. Such lasting effects imply that additional infrastructure extensions to reduce congestion on roads and crowding in public transport might have a lower benefit-cost ratio than would be the case without these impacts. The paper discusses avenues for future research, including work on the role of attitude changes, the formation of new habitual behaviour, new social norms and practices, well-being effects, and the role of Information and Communication Technologies (ICT).

A paradigm shift is taking place in spatial segregation research. At the heart of this shift is the understanding of the connectedness of spatial segregation in different life domains and the availability of new datasets that allow for more detailed studies on these connections. In this thematic issue on spatial underpinnings of social inequalities we will outline the foundations of the ‘vicious circles of segregation’ framework to shed new light on questions such as: What is the role of residential neighbourhoods in urban inequalities in contemporary cities? Have residential neighbourhoods lost their importance in structuring daily lives since important part of social interaction takes place elsewhere? How is residential segregation related to inequalities in other important life domains, in schools, at work and during leisure time? The vicious circles of segregation framework builds on the traditional approaches to spatial segregation, as well as on the emerging new research undertaken within the ‘activity space approach’ and ‘longitudinal approach’ to segregation. The articles in this thematic issue improve our understanding of how spatial segregation is transmitted from one life domain to another as people sort into residential neighbourhoods, schools, workplace and leisure time activity sites, and gain contextual effects by getting exposed to and interacting with other people in them.

To some degree, metropolitan regions owe their existence to the ability to valorize agglomeration economies. The general perception is that agglomeration economies increase with city size, which is why economists tend to propagate urbanization, in this case in the form of metropolization. Contrarily, spatial planners traditionally emphasize the negative consequences of urban growth in terms of liveability, environmental quality, and congestion. Polycentric development models have been proposed as a specific form of metropolization that allow for both agglomeration economies and higher levels of liveability and sustainability. This paper addresses the challenge of how such polycentric development can be achieved in planning practice. We introduce 'agglomeration potential maps' that visualize potential locations in a polycentric metropolitan area where positive agglomeration externalities can be optimized. These maps are utilized in the process of developing a new spatial vision for Flanders' polycentric 'metropolitan core area', commonly known as the Flemish Diamond. The spatial vision aspires to determine where predicted future population growth in the metropolitan core area could best be located, while both optimizing positive agglomeration externalities and maintaining its small-scale morphological character. Based on a literature review of optimum urban-size thresholds and our agglomeration potential maps, we document how such maps contributed to developing this spatial vision for the Flemish metropolitan core area.