Dynamic on-the fly assessment of the 15-Minute city powered by the accessibility instrument GOAT

While there is still a debate on what a 15-Minute city is, this vision raised unseen attention for local accessibility. Accessibility measures for decades aim to express the potential of our transport and land-use system. Also, the number of scholars developing indicators to measure the 15-Minute city is increasing fast. When measuring accessibility, numerous practical and methodological open questions are inherited or further complicated when measuring 15-Minute city readiness. Among them are:  

• Should accessibility be measured normative or positive?  

• Accessibility to which destinations should be measured?  

• Is the indicator flexible enough to answer diverse planning questions?  

• Should a composite indicator or several indicators be used?  

• What is the acceptable travel time to certain destination types, and what is a suitable travel time cutoff?  

• On which spatial resolution should the indicators be computed?  

While the list is incomplete, this work does not aim to find exact answers to them. Instead, it proposes a flexible set of cumulative and gravity-based accessibility measures as a toolkit that reacts to the described challenges when analyzing the 15-Minute city readiness. It follows the understanding that one single composite indicator falls short when concrete weaknesses in accessibility (e.g., lack of grocery supply) need to be identified or interventions (e.g., new supermarket) are assessed. Meanwhile, the power of composite indicators is the seeming clarity of communication. The presented work is part of the co-creative development of the dynamic accessibility instrument GOAT, which is carried out by a group of scientists, software developers, GIS experts, designers, and planning practitioners. Based on a literature review, expert interviews, and planning workshops, the requirements for the software are iteratively derived. As a geographical study context, the Region of Munich, Freiburg, Aachen, and Görlitz was chosen, which covers urban, suburban, and rural areas.  

The basis of the accessibility measures, which are visualized as heatmaps, are generic matrices that map the travel time for the modes of walking and cycling from a hexagonal raster to diverse opportunity data sets. As opportunity data sets, various daily services from the groups: education, leisure, public transport, health, living, and shopping are integrated. A cumulative, modified gaussian, and cumulative gaussian are implemented as impedance functions. The sensitivity parameters can be adjusted for the modified gaussian and cumulative gaussian functions. For all three functions, the cutoff travel time can be between 1-20 minutes. Furthermore, a weight can be specified for each category to build composite indicators. While the sensitivity can be adjusted, an online survey with approximately 800 participants in Germany provides a suggested calibration. The analyses are presented in web maps that can react to land-use scenarios (e.g., opening/closing of a supermarket).  

Predecessors of the indicators are currently used in the planning software GOAT in practice, and the described improved version of the indicators is currently integrated. Meanwhile, the indicators were computed for the study context, and comparisons show their strengths. Upcoming studies aim to compute the indicators in international case studies and apply them to different planning questions.