1.1 The Principles of the Terroristic Phenomenon for Understanding the Threat in the Outdoor Open Areas
Terrorism is presently associated with nationalist claims rooted in extremist ideologies arising from political or religious disparities [1, 2]. These characteristics are intricately tied to the human dimension of the threat, posing challenges in parametrizing these events. As the term implies, acts of terrorism are strategically planned to instil terror, fear, and disorientation. Additionally, terrorist violence exhibits two key characteristics: a material function causing immediate physical damage and a symbolic function supporting the concept of terror on a large scale, impacting both the physical dimension of the built environment (BE) and the human dimension of its users. These characteristics are relevant especially when relating to the urban BE, where public spaces (that can be generally associated with outdoor Open Areas—OAs, such as streets, squares, urban parks, and other un-built areas in the urban fabric [3]) can be affected by dynamics in users’ attraction over space and time depending on social issues, and which can host different functions with a high level of attack desirability by perpetrators [4‐6].
The domains of “threat” and “disaster” concerning terrorism intersect with the management of critical natural events, albeit with five macro differences identified [7‐11]:
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Firstly, the significance of damage in terms of casualties and targets in terrorist attacks can have major effects when compared to natural phenomena, especially considering the extension of the areas involved (e.g., the attack in Madrid in 2004 involved different train stations to the bombing attack, causing about 200 victims and about 2000 casualties and the 2009 earthquake in the territory of italian city of L’Aquila, causing about 300 victims and about 1600 casualties).
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Secondly, the choice of target locations in terrorism is driven by human will, aiming at maximizing terror, unlike natural events influenced by statistical and probabilistic factors.
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The intentional nature of perpetrator acts in terrorism contrasts with the predictability of natural events.
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The psychological impact of terrorism can surpass that of natural disasters due to the deliberate nature of violent actions.
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Lastly, disaster mitigation approaches vary significantly between terrorism and natural disasters; nevertheless, while best practice sharing is a fundamental tool for natural hazards, related presentation and dissemination for terroristic events seem to be limited for security reasons, and mainly focused on the discussion of technologies.
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While terrorism is not a new phenomenon, its contemporary significance is closely associated with the 9/11 attack in the U.S.A., characterized by symbolism, high casualties, and intricate planning [12]. Recent events in Europe have heightened the attention, emphasizing urban resilience as a strategy to enhance the physical robustness of the BE, aligning with efforts related to natural disasters and supported by increased funding for security projects. Despite major studies and applications on terrorism assessment in cities originating from the USA, European attention to the phenomenon is more recent, prompted by attacks in Madrid (2004) and London (2005), leading to the development of national regulations for regional analysis and management of the threat, particularly in crowded, political, religious, sensitive, and public places. It is the case of the Italian regulations,1 issued after the tragic events in Torino in 2017 where a false terrorist alarm caused a rapid evacuation of Piazza San Carlo during a public event, and the German experience in managing the security for mass gathering events with organizers.2 These efforts have been then supported, at the international level, by the definition of guidelines and white books to support risk assessment and mitigation, having a special focus on public space and on architectural and urban design issues (e.g., at the European level, please compare with [4]).
The complexity of terrorism risk assessment revolves around three primary factors: defining the threat, establishing principles, and incorporating multidisciplinary perspectives.
Comprehensive encyclopaedias on terrorism indicate the absence of a universally accepted definition for this phenomenon, emphasizing the localized nature of defining terrorism in national and international regulations [13, 14]. However, three simultaneous key aspects are crucial in characterizing a terrorist act: the perpetration of violent actions aimed at causing fatalities, typically carried out by an individual or an organized group with a coordinated intent for violence, and the selection of symbolic or highly public targets. Consequently, the challenge in defining terrorism represents the initial layer of complexity in risk assessment.
The Global Terrorism Database (GTD)™ stands out as a coherent resource for collecting and managing terrorism-related events.3 Developed by the National Center for the Study of Terrorism and Responses to Terrorism (START)4 at the University of Maryland, GTD employs specific terrorism “characters” and “criteria of cruel acts” for effective event identification [15, 16].
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Additionally, understanding the logical criteria underlying the terrorism threat is a subject of discussion. In this sense, G. Woo’s work [7] represents one of the most significant researches on this topic and highlights the distinctive principles, which have been synthesized into four main terrorism principles (TPs):
TP.1.
The impact factor relates to the concept of maximizing the terrorist attack. This principle can be divided into two macro-categories:
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TP.1.1-Macro-terror is characterized by the reduction of the frequency due to the complexity of attack planning and execution weapons.
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TP.1.2-Micro-terror is characterized by less management complexity and a high probability of repeatability.
TP.2.
The “Publicity Impact is Key to Targeting” highlights the perpetrator’s need to maximize media repercussion.
TP.3.
Inter-dependence and replacement of targets in compliance with the principle according to which “terrorists will attack the softer of two similarly attractive targets”. This principle can be divided into two macro-categories related to protection systems:
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TP.3.1-Hard targets, such as government buildings or military headquarters, concern buildings characterized by a system of active or passive protection technologies, regardless of the probability of occurrence. Professionals and relevant political, religious, or media figures belong to this class.
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TP.3.2-Soft targets, including subways, pubs, other public spaces as well as vulnerable sites without any type of defence measure against these phenomena. Considering the human relevance aspect, it refers to the community, gathered in extensive urban areas, lacking effective protection systems from attacks.
TP.4.
The characterization of terrorist weaponry, relating to the criterion of minimizing resistance, facilitates the evaluation of the level of threat and the equipment type used by the perpetrator. The same prefers traditional and easily available weapons (guns and explosives).
Thus, TPs highlight relevant aspects of the attack goals which include the maximization of the attack impact, the importance of publicity for targeting, the interdependence and replacement of targets, and the characterization of terrorist weaponry.
Despite this analysis of the human phenomenon, the multidimensionality of the terrorism threat is explored in the literature through mono-thematic and detailed studies, covering simulations of human behaviour, economic analyses of losses, countermeasures, and specific attack types about critical urban infrastructures or specific case studies [17‐30]. On the other hand, there is a gap in the general risk assessment and multi-temporal management of urban areas, leaving certain aspects unexplored.
Among public spaces, OAs surely represent a paramount class within the “soft targets” (compared to TP.3.2) in the urban BE [4]. In fact, in OAs, micro-terror may fully describe the goal of perpetrators in violent actions, while their higher proneness in suffering the re-iteration of actions can be related to the lower levels of protection that usually characterize such places in daily use.
On the other hand, the concept of OAs as a complex system of buildings, users, and infrastructures, serving and interacting within the perceived urban un-built area, requires to be analysed in depth, trying to understand which and how uses and services contribute to enhance or reduce the proneness of places, while physical features are addressed to understand the inherent vulnerabilities and use to evaluate the potential exposure of the violent events.
Starting from this, the following Chap. 2 presents the results of the phenomenological analysis of the terrorist events in European cities, trying to understand the relevance of the OAs, both as un-built areas and as a system, as soft targets within the urban extension, considering the GTD database.
1.2 The Security of Cities, the Human Factor, and the Terrorism Threat
Coherently with the main sustainable goals of the “secure and safe cities”, existing literature and experiences applied within urban BE to mitigate and reduce the hazards and effects of terroristic events offer the opportunity to understand the interrelations between the physical environment and the human factor during the events. To this end, Risk Mitigation and Reduction Strategies (RMRSs) have been already observed and translated into regulations and guidelines which serve to guide urban policymakers in guaranteeing urban security and users’ safety.
Considering major international guidelines, RMRSs can operate in two distinct modes and timeframes [31‐34]:
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Pre-event, aiming to prevent, detect, and delay emergency conditions through preventive measures or management procedures implemented by stakeholders and law enforcement agencies (LEAs);
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Throughout the violent act, where strategies must minimize casualties and facilitate evacuation with the support of LEAs and the defensive organization of the BE layout, guiding individuals to adopt safe behaviours during emergency phases.
Indeed, these issues should be correlated not only with their impact on the target desirability by the perpetrators, but also with the users hosted in the BE, who can adopt different behaviours depending on the stressors they are facing, as well as on the level of protection and safety perception given by the BE itself, the effects of the attack, and the implemented RMRSs [6, 32, 35‐37]. As suggested by previous works for different kinds of emergencies affecting the BE (e.g. fires [38]) and, in particular, the OAs (e.g., earthquakes [39]), including the “human factor” in risk assessment and mitigation can effectively support the development of RMRs also in respect to terrorist acts [35]. The behavioural design approach moves in this direction, considering the analysis of users’ exposure, vulnerability, and behaviours in emergency conditions as the key element to support such tasks.
In particular, the response to the violent act implies the interactions between the users and the perpetrators considering both the attack itself (e.g., users killed or wounded by the terrorists) and the evacuation process (i.e., users move far from the attack area to restore safety and protect themselves from the terrorists). User behaviour hence includes risk perception before the event (which can affect permanence tasks in the BE), and motion tasks during the emergency (i.e., motion speed, path selection, run-hide-cover, and also fight behaviour against the perpetrators), and previous studies suggested how they can be considered as consistently different from those noticed in other kinds of emergencies (e.g., fires, earthquakes) or general purpose emergencies and evacuation [35] (compare with Chap. 3). Understanding and modelling the user behaviour in terrorist acts can support the definition of such RMRSs, thus defining the basis for implementing a complete behavioural design approach to terrorist acts. User-oriented assessment can be then combined with issues related to the specificities of the perpetrators’ “modus operandi”, the liveability of the OAs, the applicability to specific contexts in terms of morphology, identity features (e.g., historical OAs), and intended uses (e.g., public spaces also used for mass gatherings) [36].
In that sense, it is worth noting that the overall picture of RMRSs is already well-defined for “hard targets” and specifically for special buildings or places, such as government buildings, critical infrastructure, and police stations. This cannot exclude them from their assessment for the application in “soft target” contexts, including OAs but can help the understanding of the emergency phase where evacuation safety regulations can adhere to common standards applicable to both hard and soft targets [31, 40, 41]. The study of such guidelines may support the comprehension of good practices in the emergency phase, and also the understanding of how and which physical elements and properties should be included in risk assessment and risk mitigation design for “soft targets”, aiming at a sustainable and effective design of strategies and solutions [37, 41‐45].
On the other hand, the design of solutions should consider sustainable applicability in real places, taking advantage of redundancy, adaptability, coordination, and costs as determinants to compare RMRSs and to evaluate how RMRSs combined applications can be implemented [32, 46‐49].
In that sense, combining the international experiences about risk mitigation and the phenomenological analysis of the European terrorism threat, fast methods to determine the class of risk for real OAs can represent key tools to support local administrations and their designers/technicians, assuming that they can also have a low level of knowledge on the matter (compare with methods defined in Chap. 4, mainly declined for Italian case studies). Specifically, all the properties and elements that may interact in the risk assessment should be jointly considered in order to provide a tool to compare the riskiness of real OAs and to determine the possible attack points within a place.
Moreover, the characterization of RMRSs in systems of effective, compatible, and redundant strategies can support the choice of a well-designed solution for real OAs, considering the main results of the behavioural-based assessment during the events in pre- and post-designed scenarios. To this end, according to the behavioural design approach [35], simulation-based methods can provide useful insights into the specific dynamics affecting the event with respect to the user response to the perpetrators’ actions [37, 50]. Nevertheless, in view of the similarities and differences in user behaviour during terrorist acts and other emergencies [35, 41], specific modelling tools should be developed according to experimental data (see Chap. 3), and then risk indicators should be defined to evaluate the impacts of certain attack affects and user behaviour on safety levels (see Chap. 4). Finally, applications to real-world case studies (see Chap. 5) can provide insights into the reliability and capability of this behavioural design approach.
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