European Cities Prone to Terrorist Threats: Phenomenological Analysis of Historical Events towards Risk Matrices and an Early Parameterization of Urban Built Environment Outdoor Areas

Among other risks, contemporary cities are exposed to terrorism. In addition to being sensitive targets, recent events in Europe have underlined the relevance of public open spaces (squares, streets, etc.) as particularly defenseless parts of Urban Built Environments (UBEs). Despite the fact that previous theoretical studies about radicalism have highlighted the "regional" dimension of the threat, the assessment of terroristic risk is still related to American guidelines. This creates new research scenarios for European UBEs and associated Outdoor Areas (UBEOAs). Thus, this paper provides two correlated main goals. The first is the phenomenological analysis of terrorist threats in European UBEs, starting from the events catalogued in the Global Terrorism Database. Specifically, the matrix of risk is assessed by combining (i) the main urban Environmental Classes (ECs) and their sub-classes, referred to as Outdoor Areas (OutECs), and (ii) the Attack Types (ATs) in order to determine the most efficient and recurrent combination of attack methods and targets (AT-EC and AT-OutEC). Then, the paper identifies the parameters influencing the terroristic risk of the most recurrent and efficient attacks identified in European UBEOAs, starting from (i) the analysis of inherent features of the European phenomenon, (ii) previous experiences in the literature and (iii) the permitted strategies and guidelines in European States. The main results are related to the relevance of Armed Assault and Bombing/Explosion Ats and Open Areas with the presence of public and strategic/symbolic buildings (ECs), while an interesting point of discussion is represented by physical obstacles.