Terrorist impacts have been increasing over time in many countries, being one of the most significant threats for the Built Environment (BE) and its users [1
]. Emergency conditions due to a terrorist act occur quickly and unexpectedly and are moved by the “will” of the attackers “to hurt innocent people, kill or injure them, or inflict significant damage on essential infrastructure at a single instant or over time, or plan to do so, to bring about political, religious or ideological aims” [3
]. Thus, they can be categorized as man-made destructive actions [4
]. Due to their unpredictable occurrences, they are assimilable to Sudden Onset Disasters (SUOD) [6
The more frequent environments where terrorists perform attacks seem to be urban BEs, especially if highly populated [2
]. According to consolidated approaches [11
], targets are defined in terms of: (1) quantity, such as the number and typologies of BE users, tourist presence, the economic values of a BE and hosted activities; (2) quality, by preferring strategic buildings and symbolic targets, such as cultural, religious and institutional places and their occupants. Large cities seem to be more potentially affected by terrorist acts, since here the effects can be maximized [8
]. The BE for terrorist act targets should be considered as the system of indoor (the building) and outdoor areas (the open spaces in the BE) because of their complexity in case of an attack [8
]. As for other SUODs, in fact, the outdoor area (the open spaces in the BE, e.g., streets, squares) and each facing building are characterized by layout, facilities, use, occupants’ presence and management strategies that interact in case of an emergency and so also alter the risk levels for their users [15
International classifications of terrorist targets, by including the ones of the European Commission definitions, recognize “hard” and “soft” targets in relation to the protection strategies and risk management that are applied to them [10
]. Government buildings, military institutions and additional strategic buildings are “hard targets”, characterized by codified and significant control levels (including restricted access to the public) and protection (including armed guards) measures [18
]. On the contrary, urban BEs (including open spaces) are ideally “soft targets” for terrorist acts, being characterized by a “high concentration of people, low or no security against violent attacks and attraction for the attacker” due to the exposure contents [19
]. They “may be selected by terrorists […] thus inflicting fear to the population and attaining media coverage” [13
]. Sights are an example of a significant BE at risk. In fact, such outdoor pedestrian areas (e.g., public spaces, squares, avenues) and the symbolic (historic or religious) buildings facing them could lead to a critical crowding level [13
]. Moreover, such places can also temporarily host mass gathering events (e.g., concerts, festivals), becoming very attractive for attackers “for their insufficient or minimal security measures” [10
]. In this sense, human-centred factors have a significant role in the overall risk and effects of a terrorist act, as well as of possible risk management strategies, as for other SUODs [22
]. Finally, further specific buildings could catch the attention of terrorists. For example, public facilities devoted to educational and health purposes (e.g., schools, hospitals) usually host sensible and exposed people to risks [10
]. Additionally, the same buildings can be drastically affected by potential crowding conditions.
To increase the safety of the BE, Risk Mitigation and Reduction Strategies (RMRSs) can operate in two different manners and times [8
]. Before the event, they are aimed at deterring, detecting and delaying emergency conditions through preventive measures or management procedures implemented by stakeholders and Law Enforcement Agencies (LEAs). During the attack, they are applied to reduce the number of victims and manage the evacuation with the LEAs’ support and the BE layout defensive organization, which can lead to people adopting safe behaviours during the emergency phases. In addition to this general classification, strategies applied at the single building scale are generally well codified, especially for “hard targets”. Here, RMRSs follow codified standards for counter-terrorism actions provided by governments and intelligence forces [8
]. When considering the hosted users, they also relate to common evacuation safety regulations, which are applied to both hard and soft targets [22
]. However, the coordination of RMRSs in BE application, the inclusion of human-centred issues in RMRSs definition, and the presence of holistic-based methodologies for BE stakeholders’ decision support seem to be generally poor if compared to other kinds of SUODs, such as fires or earthquakes [33
1.1. Work Aims
In view of the above, sustainable and effective solutions for risk mitigation in the BE should take advantage of the elements composing the BE itself to increase the safety levels for BE users before and during a terrorist act, by deterring it and managing its possible emergency consequences, respectively [35
]. In a sustainability-based and holistic perspective, redundancy, adaptability, coordination and costs are used to compare RMRSs and to evaluate how RMRSs combined applications can be implemented [8
]. A human-centred perspective is assumed to additionally evaluate how the RMRSs can effectively support the BE occupants before and during an emergency due to a terrorist act, also in view of the liveability of the BE in ordinary conditions. According to previous research [22
], this work considers the following leading factors for human-centred issues: (1) interactions between RMRSs and emergency behaviours; (2) individuals’ perception issues, including liveability of the BE; (3) applicability of the strategies in different users’ presence contexts, by mainly focusing on mass gatherings as critical conditions for BE use; (4) and capabilities given by behavioural simulation models in the RMRSs definition and validation process.
In agreement with these objectives, this work is structured in the following sections. Section 2
collects solid regulatory frameworks and guidelines from high-risk affected countries all over the world, and then resumes the existing classification criteria of RMRSs provided by them and in view of previous literature works, according to a classical review approach. Section 3
organizes data from the regulatory frameworks into the different elements composing the BE and its management, while Section 4
discusses them according to the sustainability-based, holistic and human-centred perspectives.
The described goals and instruments are functional and are part of the National Relevant Interest Research Project BE S2
ECURe (funded by the Italian Ministry of Education, University, and Research), in which the terrorist risk assessment in crowded BEs is combined with other threats (earthquakes, heat waves and pollution) in order to determine a performance-based approach useful in measuring the multi-risk resilience of BEs [41
]. In this holistic approach, the project and this work include human-centred factors as a key metric element, thus overcoming the limitation of current approaches, which seem to generally underestimate the influence of users’ behaviour in disasters and their relation with elements composing the BE and its management.
3. Classifying Existing Paradigms by Considering the BE Elements
Existing RMRSs classifications in Section 2
suggest that RMRSs can be assigned to different elements composing the BE and its management [46
]. This approach is consistent with the Section 2.2.5
outline and focuses on the effective elements to be planned by safety designers [13
]. Based on the general definition for physical versus management-oriented classifications in Section 2.2.5
, RMRSs can be classified considering the key factors organized by Figure 2
. In particular, they encompass:
The design of the physical elements composing the open space in the BE, by focusing on those used as perimeters (e.g., building façades, barriers in outdoor areas) and those placed inside it (e.g., outdoor areas and buildings having a specific function and/or feature) (Section 3.1
BE layout, by involving RMRSs dealing with the organization of indoor and outdoor spaces, distance-related issues (i.e., standoff), and emergency facilities (Section 3.2
Access control and surveillance, dealing with the strategies to be implemented towards such goals on the border of/inside the BE (Section 3.3
Safety and security management, evidencing how safety and security staff actions could reduce the risk before/during the threat (Section 3.4
This classification ensures the creation of a unique analysis layer on which specific actions are assigned to compose physical and management-related aspects. It succeeds in having a possible multi-purpose standpoint with respect to the target, the attack, and the time and space dependency. Nevertheless, some approaches are related to specific attack types as a direct consequence of their design principles and goals. The following classification discussion tries to evidence such issues. Dependencies between the RMRSs are discussed to evidence how they could be jointly implemented to improve the sustainability of safety and security in terrorism-prone scenarios.
3.1. RMRSs by the Design of the Physical Elements
The “safe perimeter
], the physical elements evolved towards the concept of visible/invisible solutions, adding invisible or well-integrated products or solutions to traditional heavy barriers (e.g., concrete ones) [8
]. All these solutions require discussion in terms of mitigation efficacy [32
], aiming at their qualifications in terms of:
Resistance to impacts that usually depend on vehicle typology and speed;
Geometric efficacy when solutions are a system of independent elements;
Emergency compatibility to guarantee the possibility of moving out of the BE site (e.g., correct dimensioning of passages, barriers that can be knocked out by evacuees).
The “Building shape” is mainly applied to buildings with the main purposes of:
Facing blast load effects. In this sense, these strategies firstly consider building geometry, size (e.g., horizontal development is preferred to a vertical one) and façade continuum (re-entrant corners, circular and concave forms). Furthermore, the immediate building surroundings could additionally ensure a positive effect, by using safe perimeter-based solutions (e.g., barriers against bombings, outdoor spaces plano-altimetric configuration, and so on);
Preventing possible assaults of terrorists inside the buildings (e.g., intrusion and armed assaults or following bombing as internal threats) or in the immediate surroundings. In this sense, these strategies try to ensure the possibility of blocking views of the inside assets to perpetrators or improve the building control (see Section 3.3
). As well for this attack type, the buffer zones could support such strategies while being combined with building orientation, vegetation use, building components and external area planning elements (e.g., obstruction screens and man-made hillsides).
According to existing frameworks [46
], these measures should be combined with: (1) safe perimeter
measures to reduce the risk of building occupants, by means of unoccupied or low occupancy areas in the proximity of the entrances and of the perimeter (e.g., buildings envelope); (2) management strategies, by identifying secured and unsecured areas within a building and separating them with buffer zones. Such characteristics evidence how they can be difficultly compatible in the case of application to existing BEs, unless the interventions are applied to the elements in the open spaces. Finally, the coordination between building shape
and emergency layout
-oriented strategies could increase the evacuation motion towards the shelters or the building exits, as for general fire safety solutions.
The “Façade protection
” can guarantee the limitations of threat and damages propagation from the outside into the buildings. Structural properties, in reacting to bombing attacks, assume a paramount role indeed [42
]. However, it is referred to the most vulnerable element in facades (e.g., windows, doors or structural glass) as additional protection near to the discussed “building shape
” as well as the BE layout-related RMRSs [46
]. According to existing frameworks [35
], technical details concern these ideas:
Laminated glass with an inner layer of polyvinyl butyral well secured into the frames is preferred. Concerning their position, windows are placed low down, reducing the distance of flying glass into the room.
Security doors provide enhanced protection against forced entry and overall resilience of the outer shield of the building; moreover, doors should be bomb resistant, bullet resistant and extreme-intrusion-attempts resistant.
Such characteristics evidence how they can be limitedly sustainable in case of application to existing BEs, unless the interventions are applied to the simple retrofit of the existing building components.
3.2. RMRSs by BE Layout
” oriented RMRSs can be implemented in relation to the safe perimeter
and related BE site layout, combining minimum distances with physical elements or specific area to keep vehicles away from the entrance of buildings or their more vulnerable parts [46
oriented RMRSs are specifically adaptable to increase the distance between bombing sources and specific targets.
” RMRSs are a human-centred solution aimed at protecting the BE occupants in safe areas placed as close as possible to their position before the attack. Their efficiency depends on the implementation of proper “emergency layout
” and “emergency plan
” measures as well as on “structure
” oriented measures to contrast the attack-related damages [57
]. Such strategies are borrowed from emergency plans concerning fires or climate-related events (e.g., storms, hurricanes) and they are usually identified as safety areas located inside the buildings [34
]. In fact, “sheltering” inside buildings (as an “invacuation” or “inward evacuation” procedure) can be a valid approach for outdoor areas in the BE in the case of attacks due to external threats in respect to the buildings themselves, such as vehicle attacks, armed assault or even low-level risk of chemical, biological and radiological (CBR) attacks [42
]. The effectiveness of such RMRSs can be guaranteed when combined with:
The design of physical elements in the BE, adopting “Façade protection” measures to contrast the attack itself;
The aforementioned “emergency layout” strategies by integrating them with the support of management solutions. In this sense, additional RMRSs can mainly concern: (1) correctly signalling the sheltering areas by means of signs or by using “emergency plan” related actions by safety and security staff members; (2) also promoting “users’ involvement” before the event;
Additional “access control and surveillance” solutions to impede the attackers who could arrive at the shelters.
” and “emergency layout
” related measures can be guaranteed by correctly applying the RMRSs relative to “safe perimeter”
. Through their implementation, the open spaces in the BE should be divided into sectors able to host a definite number of occupants, and they are fully designed in mass gathering events. Furthermore, the contribution of each outdoor and indoor element towards this analysis should be considered. Specific areas should be circumscribed when locating emergency facilities, and access and exit points should be identified with well-delineated borders by controlling the crowd flows [32
]. “Emergency layout
” strategies can also limit critical phenomena in crowd motion to reduce time-increasing and individuals’ safety-decreasing man–man interactions, such as counterflow movements along the evacuation paths or high-density crowd conditions [38
3.3. RMRSs by Access Control and Surveillance
” is a paramount strategy for hard targets or in case of mass gathering events due to its strongly deterrent character for terrorist attacks [9
]. Safety personnel are often employed to manage such controls by following specific management procedures. If combined with safe perimeter solutions (i.e., heavy barriers), they constitute the most invasive solutions [8
]. However, novel technologies could be employed to speed up the access controls and to make them less invasive, such as body scanners, metal detectors and optical devices for counting people. The use of video surveillance systems (CCTVs), distributed across the overall BE, can support the investigations of intelligence authorities to prevent possible terrorist attacks and to detect the perpetrators after the event. In addition, the effectiveness of these two RMRSs are strictly influenced by: (1) the application of reliable coordination
actions; (2) the robustness of the infrastructure which collects and disseminates data between the First Responders and the LEAs [11
In addition to these two main control and surveillance RMRSs classes, “illumination
” represents a real and psychological deterrent for continuous or periodic observations by an aggressor, as well as a low-cost solution [46
]. Different illuminance conditions can be designed to be increased over time, while site lighting can be helpful as a response to different levels of alert or to support users in attack-affected conditions (e.g., proper illuminance of the emergency layout
). Finally, security illumination tools should be combined with CCTV since the cameras may need high intensity, low intensity, or infrared light for proper operation.
3.4. RMRSs by Safety and Security Management
” mainly include LEAs, Security Forces, and other surveillance bodies in soft targets. They should perform all the actions mainly related to security issues [7
] that are: (1) before the event, to mainly deter and detect possible attackers; (2) during the attack, to mainly support First Responders in “first aid
” operation in the immediate aftermath and to detect the assaulters. In a “coordination
” perspective, they should be adequately trained to face effective conditions, and they also require the implementation of effective access control and surveillance RMRSs [32
]. From a procedural point of view, while responding to the attack conditions, RMRSs concerning “security personnel
”, “first aid
” and “coordination
” should also be strongly supported by the emergency plan
and emergency layout
. In addition, tools to estimate the damage caused by the attack itself by including both direct fatalities due to the event and crowd-related and behavioural-based phenomena (e.g., crushing effects in the crowd) can support the combined planning of such RMRSs [38
In view of the above, it is important to underline the role of two main RMRSs. Firstly, the “emergency plan” becomes a key element in management oriented RMRSs. This is the sum of subsequent indications for emergency personnel and users about the employment of the predisposed emergency facilities. Therefore, emergency plans have to be strictly related to the other RMRSs and able to organize and create interactions among them. Emergency coordinators and designers should have the capability to comprehend every key aspect and potentiality of each emergency measure and to take advantages from them by putting each RMRS into communication, to prevent and discourage the attack and to work jointly after the attack. Emergency plans have to take into account the main features of soft target typologies, which depend on their intended use. In particular, they have to face with the possibility that specific activities take place, and, in the case of mass gathering events, with the expected number of people (which can sensibly vary in soft targets and, especially, in outdoor areas).
It is worthy of notice that such measures are well codified in the literature when they are referred to a single part of the BE (e.g., a single building) or to the BE as a whole during specific events, such as organized mass-gatherings, festivals, and so on. In the case of “daily” use of the BE, it cannot be possible to define an overall system for safety and security management. Nevertheless, it is important to evidence how future efforts in defining common action plans between all the BE stakeholders will improve the coordination of counterterrorism measures in this sense. Secondly, the “user’s involvement” concerns all the actions aimed at improving the awareness, the preparedness and the correct response of citizens to the risk of possible terrorist attacks, as for other kinds of SUODs [44
]. In fact, the promotion of “educative” initiatives (e.g., booklets or guidelines) to common people can improve their awareness and preparedness in case of necessity (before the event) [62
]. Some European countries (i.e., Belgium, United Kingdom, Germany) have supported the development of counterterrorism measures for individual devices (mainly smartphone applications). Some of these apps constitute a tool to provide detailed indications of the right behaviours to perform linked to the types of terrorist attack, such as the virtual platform iNFO-R!SQUES.be (available at: https://www.info-risques.be/fr
; last access: 30 September 2020). Other apps inform users about ongoing terrorist attacks, integrating the alert message system to the users’ location thanks to the Wireless Local Area Network for smartphones, such as the KATWARN mobile application (available at: https://www.katwarn.de/en/system.php
; last access: 30 September 2020).
4. Challenges in a Holistic, Sustainable and Human-Centred Approach to Risk Reduction Strategies’ Classification and Evaluation
The brought classifications of existing RMRSs are not enough to determine which RMRSs are more convenient to be applied to a specific case study. Therefore, the main challenges for risk mitigation and management solutions have to be assessed from a sustainable point of view as well. Hence, RMRSs should be oriented towards the following main sustainability criteria here synthesized [8
Moving towards redundancy criteria of the resilient BE by combing different strategies to ensure that each of them could support the risk-reduction process (according to different operational procedures) in all the phases of the disaster;
Selecting solutions to be effective for more than one terroristic threat/attack typology;
Adopting a human-centred approach to include the behavioural reaction of the exposed individuals (especially in crowds) and of the terrorists, also in respect to the human–BE interactions (i.e., for the promotion of correct emergency behaviours);
Including mass gathering conditions during strategy planning to ensure the safety and security aspects of different BE use situations;
Considering the possibilities of connecting different BEs (at a local scale, e.g., indoor-outdoor; at a global/urban scale) to face the disaster;
Promoting a psychological function of the strategies to ensure they are perceived as reliable by the citizen, to deter the terrorists but also to guarantee the liveability of the BE under normal use conditions.
A critical evaluation of such aspects is outlined through Table 5
and Table 6
, which focus on the general applicability, adaptability and costs approach issues. In particular, redundancy criteria and coordination criteria in Table 5
and Table 6
are expressed according to the correlation in Appendix B
and Table A3
, respectively). Furthermore, Table 7
and Table 8
deal with human perception and evacuation simulations, according to the human-centred standpoint.
Some remarks on research objectives, future challenges and design/policy/planning perspectives can be summarized according to this critical overview of RMRSs and their classification.
Firstly, according to this work’s aims, the proposed classification critically reviews, for the first time, current consolidated RMRSs, filtering them through the different elements composing the BE and its management. The proposed classification succeeds in evidencing the complex relation system between all the physical and management-related elements composing the BE (outdoor and indoor spaces). This result is achieved by considering solid regulatory frameworks and guidelines. The findings highlight how the BE and its users have to be considered not as the attack’s background, but as an integral part of the RMRSs themselves. In fact, safety planners should coordinate (1) the BE layout design, which can support space organization in normal fruition (i.e., standoff distances; area and access control by the stakeholders) and in emergency conditions, with (2) BE-oriented interventions to establish safe perimeters and adopt constructive solutions for the protection of buildings’ parts, façades, and structures (to limit the terrorist acts’ effects).
The organization of RMRSs, depending on the elements composing the BE and its management, can represent a unique and easy-to-use list of solutions for safety planners and decision-makers. Thus, such a list can increase their awareness in respect to the implementation of such solutions in the BE in a sustainable and quick manner, additionally pursuing a high-level of liveability of the BE. Finally, analyses on the combination/coordination of RMRSs to contrast a terrorist act can support safety planners and decision-makers, also in view of the use of behavioural-based simulation tools, as remarked by Table 7
and Table 8
Furthermore, the proposed classification list can be used to collect data from real-world BEs. Such an action will define recurring conditions in RMRSs’ implementations depending on the frequency of RMRSs implementation. Widespread data collection in national and international contexts will support the definition of typical BE scenarios depending on the applied RMRSs.
Secondly, the proposed classification and the human-centred principles codified in Table 7
and Table 8
support the combination of RMRSs to manage specific emergencies, especially related to actions carried out by the BE users. In fact, they can sustain the organization of future regulatory frameworks which can manage terrorist risks for the BE under different scenarios of use of open spaces and facing buildings. In particular, in the case of open spaces where mass gatherings can occur, the results remark how the access points should be controlled, adopting security services through trained and adequately coordinated personnel.
In this sense, future challenges in RMRSs’ definition and assessment will move towards a human-centred perspective, as for other kinds of SUOD such as fires, so as to check how the BE users’ control and support can be achieved in all these different scenarios of BE use. Current RMRSs solutions to increase people’s safety against terrorist attacks are limitedly based on data nearer to reality [40
]. On the contrary, many simulation models were developed in the past, and they also included the effects of the terrorist acts on the BE, but most of them adopted general purpose behaviours (e.g., those related to fire egress) and only a few of them were based on effective evacuation scenarios from terrorist acts [40
]. Such modelling issues can beneficially assume microscopic approaches due to the possibility of representing the interactions among attackers, attack actions and occupants over time and space (e.g., by pursuing a social-force based [67
] or a cellular automata [66
] approach, also integrating agent-based models [71
]). Thus, further efforts to provide reliable databases on terrorist act-related emergencies are urgently needed and they should include the possibility of simulating the effects of the adopted RMRSs [33
]. In this way, these studies could fully include a human-centred approach into RMRSs’ planning practices to support decision-makers [31
]. The database outcomes could be used for preliminary activities related to the development, evaluation and validation of behavioural emergency evacuation simulators for terrorist attacks in the BE, especially if related to specific case studies. The results of future simulations will allow us to evidence the possible interactions among the effects of the attack (and the attackers, too), the RMRSs adopted, the BE and its modifications, the crowd and the first responders in view of such an emergency [40
]. Pedestrians’ evacuation flows will be combined with analyses on the evacuation time and the causalities’ number to evidence the effects of such interactions and to inquire about the effectiveness of proposed RMRSs, thus improving community resilience aspects in specific scenarios. Simulation tools could also be applied to typical BE scenarios organized depending on the RMRSs analysis in real world contexts. They will define recurring conditions in the emergency phases, depending on the BE features (e.g., morphological-constructive features), the BE use and crowd presence, the type of attack and the implemented RMRSs. These activities will move towards a parametrization of risk levels and evacuation conditions in significant scenarios, thus being a quick support for preliminary effectiveness evaluations from decision-makers.
Thirdly, the RMRSs discussion through sustainability criteria is based on redundancy, adaptability, coordination, and costs, and seems to trace which RMRSs can be more convenient in relation to the obtained effects, as also remarked by Table 5
and Table 6
. In particular, the combination between safety and security management strategies and access control and surveillance seems to lead towards the highest sustainability levels, thus being a possible focus for effective RMRSs in the future, in each kind of BE. Such an approach will support decision-makers while organizing plans on terrorist act mitigations to be extensively applied in different urban BEs. Nevertheless, one of the greatest challenges for policy planning concerns the overcoming of limitations due to the combination of public-oriented actions and classified risk management strategies. As for other kinds of disasters, such as fires, earthquakes or floods [33
], strategies based on risk awareness and preparedness should be shared with the BE users to increase their safety [32
], but they should be arranged in respect to the reliability of security measures. In fact, these security measures widely concern intelligence activities and should be hence protected by public and uncontrolled accesses. The decision-makers can use simulation tools to evaluate the acceptable threshold for such safety-security measures concerning: (1) pre-disaster actions in terms of the spreading of emergency plans, which could also be known by the potential attackers; (2) and the emergency response actions to support the population during the event and the evacuation process. The level of shared information will be managed to limit the possible support to terrorists in collecting sensitive data, but it will demonstrate the level of safety of the BE they would attack, thus also becoming a dissuasive element in terrorism fighting.
Finally, the sustainability and holistic perspective-oriented criteria discussed in Table 5
and Table 6
could also be adopted to provide the bases of key performance indicators to assess the RMRSs feasibility and reliability in BE application. According to the Table 7
and Table 8
insights, the introduction of typical factors relating to the levels of support to BE users in emergencies and to the mass gathering conditions in the BE will boost this assessment process. From this point of view, the proposed approach also represents the first step towards the organization of holistic BE resilience metrics to also be applied to other disasters affecting the urban BE. However, they will be based on the elements composing the BE and its management, and they will adopt the same sustainability and human-centred key factors. In this sense, a combined analysis of the BE under several threats will be pursued by using common key performance indicators, in accordance with the goals of the wider project this paper is part of [41
]. However, the next works should provide quantitative indexes for merging them by considering different priority levels depending on each proposed criterion.
Terrorism acts have become a more frequent menace for society nowadays. Attacks mainly occur in the open spaces of our cities which are habitually frequented by inhabitants. Thus, measures able to face such sudden onset and man-made disasters have to be organized in strict relation with the Built Environment features. This work traces a critical review on existing and consolidated Risk Mitigation and Reduction Strategies (RMRSs) based on the codified regulations that risk-prone countries have adopted in the last few decades.
Classifications of RMRSs are provided according to main literature-based criteria and involve consolidated regularity frameworks. In particular, the analysis provided a distinction between hard and soft BE targets, where active and passive actions can be applied by considering their level of visibility in respect to possible aesthetic and preservation issues of the BE. The identification of the main attack typologies is supported by the RMRSs’ effectiveness depending on time and space, since different attacks imply different effects on the exposed individuals and modifications of the BE. A layer-based discussion of RMRSs’ implementation in the BE evidences the roles of perimeter elements, open spaces in the BE (and their furniture) and building components/buildings as the inner level of safety.
In view of the above, the collection of the main paradigms regarding physical and management-oriented aspects in the BE can summarize all these issues from a holistic standpoint. It provides the basis for the discussion of consolidated RMRSs based on sustainability and human-centred criteria, which essentially encompass redundancy, adaptability, coordination, costs and BE users’ support in emergency conditions. Filtering RMRSs by the elements composing the BE and its management provides the basis for the future organization of these solutions from a parametric standpoint, which depends on the effective scenario where they will be placed. Meanwhile, among the adopted criteria, the human-centred ones highlight how simulation-based approaches could play a pivotal role in assessing the effectiveness of RMRSs, also including emergency management topics, due to the significant impacts of man–man and man–environment interactions in the immediate response phase.
Future works will have to improve the dimension of the sample of the documents concerning the RMRSs by including those from other countries prone to terrorist acts. This action will allow detecting if additional measures can be included in the RMRSs classification, according to local experiences and other codified policies. At the same time, a deeper focus on case studies from real-world BE applications could support the delineation of a general theory for RMRSs’ selection and evaluation. The proposed BE-oriented classification criteria of RMRSs could be applied to real-world scenarios. These activities could delineate common implementation strategies in real contexts. Furthermore, the response of each deployed RMRS before and during the attack could also be assessed if considering BEs affected by terrorist acts. Thus, stakeholders and researchers could move towards experimental-based analyses on their effectiveness to be compared with feasibility, risk assessment and simulation-based evaluations.