Keywords: premeditated destructive events, terrorist attack, fuzzy logic, CPTED, safety, expert systems.

This paper introduces a methodology that uses Crime Prevention Through Environmental Design (CPTED) and fuzzy set theory to assess the perception of building safety against Premeditated Destructive Events (PDEs). People's perception of building safety is built upon their prior knowledge of the environment, the design of the building, and their personal experience. The prior knowledge about the environment creates user's expectation before entering the facility, and this expectation is joined with the design of the building and user's personal experience to form the user's perception of safety.

The bombings the Alfred P. Murrah Federal Building in Oklahoma City in April 1995, and plane attacks on the Pentagon and World Trade Center skyscrapers in New York in September 2001 indicate that such buildings can be destroyed anytime and anywhere. Each bombing caused the collapse of buildings, subsequently killing hundreds or thousands and shocking the American public and fundamentally changing their perception of safety from PDEs.

An expert advisor program is created that addresses the user's perception of building safety from PDEs, This expert adviser program is an effort to summarizing the assessment of the perception of building safety based on heuristics in the form of IF-THEN rules. Three tasks in building this system are performed: Analyzing the subjects in using environmental designs to prevent crimes and PDEs, creating a rule-base that contains the expert knowledge, applying fuzzy set theory in determining the perception of building safety. This advisor uses environmental variables that are defined in accordance with the CPTED methods. The goal of CPTED is improving the intended function of space by creation of a physical environment that reduces the incidence and fear of crime. Although CPTED originally was not intended as a counter-terrorism method, because of its capability to reduce the fear of crime in an environment, it has the potential to disarm one of a terrorist's most powerful weapons: fear. In this paper, the investigators adopt CPTED methods to counter-terrorism. However, the consequences of terrorism are more severe than most criminal acts. In crime prevention, the objective is to reduce the number and severity of crimes. With terrorism, a single incident can psychologically devastate an entire country. CPTED is based on three principles: natural access control, natural surveillance, and territorial reinforcement. Natural access control uses a screening process to restrict the unidentified access to the facility. Natural surveillance deters the criminals by increasing the perception of risk. The territorial reinforcement is used to create the perception of the user's ownership of the facility. The physical design can extend the user's sense of territory to identify strangers or potential criminals

Fuzzy logic and fuzzy set theory is used to establish a quantitative approach to show that the environmental changes to the constructed facilities will influence the behavior of the potential criminals and users' perception of safety. The evaluation and validation of the perception of being a safe in a constructed facility is a crucial part of applying these CPTED principles. Fuzzy set theory and fuzzy logic are tools that are capable of examining and evaluating the perception of safety of a constructed facility because unlike other conventional quantitative analysis, these tools incorporate experience-based assessment and subjective knowledge expresses using linguistic terms. Hence, this approach is more suitable for safety assessment in which imprecise information is often used.

A simulation model was developed to assess the performance of the constructed facility. There are two major tasks in building this model: the first is to identify CPTED variables. To prevent PDE from occurring, the possible CPTED variables that are specifically related to the PDE must be established and analyzed. The second task is to design and develop the fuzzy mechanism that includes the fuzzification, inference engine, and knowledge base. The inference engine is the "brain" that decides how the instances of the variables interact with each other. This inference engine collects the information and matches this information with the rules in the knowledge base. The knowledge base includes many implication rules that store the experts' experience in this field. To clarify the use of the model, the authors present an illustration involving scenarios in which users are able to develop their own rules regarding the notion of safety associated with variables contributing to such safety. Then, users enter their assessment on the adequacy of these variables to attain the overall building safety from PDEs. The perception of building safety from PDEs is difficult to evaluate without the use of fuzzy logic. Because the occurrence of PDEs is difficult to prevent with limited resources, the use of CPTED designs such as natural surveillance, access control, and territoriality can be relatively simple, inexpensive, and effective. This CPTED program will be a useful tool for planning and maintaining the safety of constructed facilities as well ongoing construction projects. Unlike many control programs that are based on physical changes made to the targeted structure, this program attempts to explore the possibility of discerning the perception of safety that is invisible to the users and to provide a logical inference tool for evaluation of the targeted facilities. The use of a VR model provides a better understanding for those who do not have experience in this field.

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