**1. Introduction**

Today, cities are at the forefront of increasing urbanization and digitalization pressures [1–3], where they play a crucial role in supporting the transition towards a sustainable and smart urbanism practice [4–6]. The current urban context is associated with numerous economic, social and environmental issues, such as waste managemen<sup>t</sup> [7–10], energy efficiency [11–13], renewable

energy sources [14,15], water managemen<sup>t</sup> [16], social, cultural, and health aspects [17–22], material flows [23], biodiversity [24], transport [25,26], land use optimization [27], air and noise pollution prevention [28–30], infrastructure mishaps [31], economic growth [32]. Policymakers, thus, need a paradigm shift, developing innovative, sustainable, and intelligent solutions to optimize urban processes and improve citizens' quality of life and sustainability of the city [33–36].

In recent years, the notion of 'sensor city' has emerged as a response to the future challenges of growing urbanization and datafication [37,38]. This new version of the city—i.e., City 4.0,—(thanks to the urban dashboards and platforms integrates Internet-of-Things (IoT) infrastructure [39], sensors [40], real-time monitoring stations [41,42], digital cameras [43], actuators [44], real-time tracking systems [45,46], big data analytical techniques [47–49], information and communication technologies (ICTs) [50,51], cloud computing [52], smart grid [53,54], artificial intelligence (AI) [55–57], autonomous shuttles [58], and other digital appliances with physical objects that characterize urban context) improves the efficiency of resources usage.

Specifically, city dashboards accommodate visual/graphical and dynamic analysis suite capable of holistically combining urban infrastructures to view, integrate and communicate real-time information on performance, trends, and future urban scenarios [59,60]. These dashboards are characterized by a high degree of interactivity with users, capable of combining, filtering, querying and overlapping large amounts of urban data [61]. Indeed, urban dashboards are implemented to facilitate an understanding on major urban issues and provide stakeholders with a sense of accountability and engagemen<sup>t</sup> on smart urban governance activities [62–64].

According to the forecasts of the "World Smart Cities Spending Guide" provided by the International Data Corporation [65], the total expenditure for smart urban solutions this year alone amounted to almost USD 124 billion. This is an increase of 18.9% compared to 2019 [65]. Specifically, global cities such as Singapore, Tokyo, New York City and London occupy the top of the ranks in terms of investments in smart urban initiatives [65]. Furthermore, cities such as Toronto, Adelaide, Hamburg, Kansas City, Dallas and Stockholm (see Table A1 in Appendix A) have implemented participatory and intelligent platforms that use ICTs to connect companies, local authorities, universities, start-ups, citizens, associations, and so on in order to support the decision-making process, allowing the collection, processing, monitoring and analysis of large amounts of urban data [66–68]. In this sense, Bibri [34] described cities as complex networks of holistic relationships that integrate smart and sustainable solutions in order to provide a suitable context for long-term urban strategic development.

Indeed, the rapid and pervasive development of ICTs taking place all over the world is transforming cities into centers of economic, social, environment and technological development with the aim of providing increasingly efficient, sustainable and smart urban services [37,50,69,70]. In this regard, the United Nations (UN) Agenda 2030 defines ICTs as necessary tools to facilitate the transition towards sustainable development [71]. Hence, policymakers use data and information sharing systems through IoT technologies for planning, monitoring, and evaluating the performance of urban policies, and for improving transparency, active participation of citizens and awareness of urban issues [72–74]. For example, cities such as Singapore, Zurich, Oslo, Geneva, Copenhagen, Auckland, Melbourne, Taipei, Helsinki, Bilbao and Düsseldorf represent forward-looking cases regarding the use of ICTs in the urban area, occupying the first places in the ranking developed by the IMD World Competitiveness Center Smart City Observatory [75].

The current urban theoretical and managerial debates increasingly focus on the role of ICTs and their integration with various aspects related to sustainable urban development [76,77]. Indeed, the literature provides several synonymous for sensor city such as digital city [78–80], smart city [81–83], ubiquitous city [84–86], knowledge city [87–89], intelligent city [90,91], techno-centric city [92], creative city [93,94], sustainable city [95,96], informational city [97,98], smart sustainable city [99–102], and artificially intelligent city [57], which express the importance of ICTs in the managemen<sup>t</sup> of the cities of future.

Nevertheless, technology alone cannot be a panacea for all urban issues related to growing urbanization [103–105]. In particular, cities excessively connected to IoT and big data analytical

solutions have often been criticized for being too techno-centric and for underestimating social and environmental aspects [106–108]. At the same time, sustainable cities struggle to integrate the technological approach with the social, environmental and economic dimensions of sustainable development [102,109]. Specifically, the use of advanced techniques (e.g., real-time monitoring stations for energy consumption, location systems to guide urban tra ffic, cloud computing systems for sharing sensitive data between governmen<sup>t</sup> departments, urban infrastructures such as smart bins, smart street lamps, and surveillance cameras) highlights a multitude of challenges related to the quality of the data used, level of protection of traditional and cybernetic urban security, necessary data integration between the various urban infrastructures, and ability to transform feedback from citizens and other stakeholders into innovative urban policies. Consequently, sensors and related ICT infrastructures are rapidly gaining strategic importance for sustainable and disruptive urban development [110]. They are not only enhancing in terms of technological aspects, but also social, environmental, and economic ones.

This paper aims to explore the main challenges related to sensor cities, emphasizing the opportunities and critical issues of this growing datafication of urban contexts. In this sense, an integrated and holistic framework is proposed which includes a theoretical and managerial review of the main disruptive technological applications. The study, thus, identifies and compares IoT solutions based on sensors, big data analysis and other technologies related to ICTs adopted by di fferent cities, to manage urban development in an innovative and computerized manner. This paper generates insights from the current sensor city best practices by placing some renowned projects, implemented by Huawei, Cisco, Google, Ericsson, Microsoft, and Alibaba, under the microscope. With the objective to o ffer a detailed overview, the paper adopts a mixed research approach able to integrate a literature review and an in-depth analysis of several case studies. In this regard, the proposed framework provides users with greater knowledge and awareness of sensor cities' development.

The paper is structured as follows. Section 2 introduces the mixed methodological approach used. Section 3 analytically describes the proposed framework, highlighting the technological factors such as IoT, big data analysis, AI, ICTs, real-time monitoring stations, sensors, cloud computing, digital platforms, and urban challenges that characterize sensor cities. Finally, Section 4 provides conclusions and some considerations on the contribution of sensor cities to future urban challenges.

### **2. Materials and Methods**

The development of this study is structured according to a mixed approach, characterize by a literature review and a detailed analysis on several case studies, in order to create a framework for policymakers that collect the forward-looking ICT urban initiatives, emphasizing a data-focused method in the assessment of urban development. The implementation of this study is carried out in di fferent phases and in this sense the methodological approach followed is illustrated in Figure 1.

In Phase 1: Identification, the research question, keywords, and research databases are defined. Regarding the objective and the research question, the study aims to identify and analyze the main characteristics of sensor cities, highlighting the impacts of their technological applications on urban development.

The attention of the study has focused that have provided an interdisciplinary and/or transdisciplinary perspective to the development of sensor city. Specifically, these scientific disciplines include technology and innovation management, urban planning, policy, sustainable development, environmental management, data science, urban informatics, geography, urban development, strategic management, and urban statistics. Moreover, the paper—through a qualitative approach—analyzes several sensor cities considered successful examples of disruptive urban actors, capable of integrating sensor strategies with the economic, social, and environmental aspects of urban development in detail. As a result, the study includes peer-reviewed journal articles, book chapters, and conference proceedings, grey literature such as governmen<sup>t</sup> documents, and industry technical reports. In terms of databases, ScienceDirect, Google Scholar and cities' websites were utilized to achieve the analysis. Furthermore, the keywords searched include (("sensor city" OR "sensor cities") AND ("ubiquitous

city" OR "ubiquitous cities") AND ("digital city" OR "digital cities") AND ("real-time city" OR "real-time cities") AND ("sentient city" OR "sentient cities") AND ("intelligent city" OR "intelligent cities") AND ("data-driven city" OR "data-driven cities") AND ("smart city" OR "smart cities") AND ("sustainable city" OR "sustainable cities") AND ("sustainable development") AND ("smart urban applications") AND ("urban IoT") AND ("urban sustainability") AND ("urban development") AND ("big data applications") AND ("urban sensors") AND ("knowledge city" OR "knowledge cities") AND ("disruptive urban development").

**Figure 1.** Methodological approach adopted (source: authors).

In Phase 2: Screening, the review aims to provide a clear and comprehensive definition of the concept of sensor city, introducing urban dimensions (e.g., governance, economy, environment, mobility, people, and living) and technological solutions (e.g., IoT, sensors, AI, ICTs, big data analytics) necessary for its implementation. In order to further refine the research, all the selected sources were screened following a set of inclusion and exclusion criteria, in line with the objective and the research question of the study. As exclusion criteria, sources with partial information and inconsistent with the topic of the study were not included in the search.

In Phase 3: Result, the integration between literature review and case study analysis provides a detailed framework useful for developing a theoretical approach to sensor cities, underlining the holistic relationships between urban sustainability and computerization aspects. Nonetheless, it is important to consider that sensor cities taken into consideration in the case studies analysis use non-uniform terminology and lack detailed quantitative data on sensor infrastructures. In this sense, a greater understanding and awareness of urban sensing is needed to develop forward-looking projects in line with future urban challenges. This research focused on 20 sensor cities, with a high quality in social, economic, environmental and technological infrastructures. Consequently, the sample analyzed does not represent the different types of cities globally. Most cities around the world are not equipped to collect, monitor, analyze and evaluate urban performance through innovative platforms and dashboards. In sum, this review provides: (a) A clear definition of sensor city; (b) A sensor city framework; (c) A detailed analysis of several sensor cities, and; (d) Various different sensing policy and actions that currently policymakers have in place.
