Cities are complex systems connected to economic, ecological, and demographic conditions and change. They are also characterized by diverging perceptions and interests of citizens and stakeholders. Thus, in the arena of urban planning, we are in need of approaches that are able to cope not only with urban complexity but also allow for participatory and collaborative processes to empower citizens. This to create democratic cities. Connected to the field of smart cities and citizens, we present in this paper, the prototype of an urban digital twin for the 30,000-people town of Herrenberg in Germany. Urban digital twins are sophisticated data models allowing for collaborative processes. The herein presented prototype comprises (1) a 3D model of the built environment, (2) a street network model using the theory and method of space syntax, (3) an urban mobility simulation, (4) a wind flow simulation, and (5) a number of empirical quantitative and qualitative data using volunteered geographic information (VGI). In addition, the urban digital twin was implemented in a visualization platform for virtual reality and was presented to the general public during diverse public participatory processes, as well as in the framework of the "Morgenstadt Werkstatt" (Tomorrow's Cities Workshop). The results of a survey indicated that this method and technology could significantly aid in participatory and collaborative processes. Further understanding of how urban digital twins support urban planners, urban designers, and the general public as a collaboration and communication tool and for decision support allows us to be more intentional when creating smart cities and sustainable cities with the help of digital twins. We conclude the paper with a discussion of the presented results and further research directions.Sustainability 2020, 12, 2307 2 of 17 representations of material or immaterial objects, e.g., machines of the real world. Digital twins enable comprehensive data exchange and can contain models, simulations, and algorithms describing their counterpart, including its features and behavior in the real world [2]. To enhance real-life perception, they can be implemented in virtual reality (VR).To that end, we developed an urban digital twin applied to the town of Herrenberg in Germany, which can be applied and visualized almost seamlessly across all scales, on multiple layers, and in different categories of data in virtual and augmented reality (VR, AR) for collaborative and participatory processes, focusing on urban planning, urban design, and decision support.The Herrenberg digital twin differs from other simulation-based studies in the field of smart cities, in particular by linking and combining various urban data from models, analysis, and simulation and by the implementation of social data collected from citizens. Furthermore, the visualization in virtual reality ("virtual twin") not only enables broad citizen participation but also collaboration between stakeholders. The advantage of using virtual reality environments-s...

Bill Hillier’s space syntax method and theory enables us to describe the spatial properties of a sustainable city. Empirical testing of the space syntax method over time has confirmed the capacity and innovativeness of analyzing spatial relationships with the purpose of understanding and explaining the socio-spatial organization of built environments. However, the conceptual framework of space syntax elements is scattered around in various academic writings. This article, therefore, gives a holistic and compact overview of the various concepts that are used in space syntax, from its basic elements to various analytical techniques and theories. To achieve this compact overview, we reviewed all space syntax literature accessible since the 1970s for finding core references to various concepts used in space syntax. Following a short description of its foundation and evolution through the work of Bill Hillier, we explain its basic concepts and measures in the form of an extended glossary. Explanations are enriched with various space syntax analyses and scenario testing on various scales that were applied to the city of Rotterdam in the Netherlands. We conclude with a discussion about the advantages and limitations of space syntax and about how this method adds value to the creation of sustainable cities.

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The global trend towards the growth of urban population poses the issue of the public space influence on outdoor practices of different subjects. Urban public space is the carrier of an ever-increasing number and density of users. Walking, both as a form of transportation and as a vector to physical activity and social interactions, emerges as a fundamental category of behaviour. The proposed research develops a theoretical and methodological framework for assessing walkability of the urban space. Building on concepts of capability and affordance, walkability is herein conceptualized in terms of the effect of affordances incorporated into the built environment on individuals' propensity to walk for different purposes. The methodological framework aims to capture the characteristics of walkability across scales as the product of the intrinsic and extrinsic properties of an urban setting.

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