Intrusion or burglar alarms have been around for more than seventy-five years (Michael 1931). Commercial establishments such as banks use them to protect their vaults. Airports use alarms to control and restrict access to secured locations. Residential homeowners use them to guard against home burglary. At issue is whether alarms can prevent crime.

Security systems can be categorized as either access control or perimeter control systems (Kobza and Jacobson 1997). Access control refers to gate-keeping that limits access to a designated area to authorized personnel. For example, in 1991, the Federal Aviation Administration (FAA) defined the Security Identification Display Area (SIDA) with requirements for use of airport identification badges in designated areas. Following the tragedy of September 11, 2001, when hijacked airplanes crashed into prominent buildings in the United States, the FAA regulations were updated significantly. Worldwide, most modern airports use an identification badge as an access control system card.

Access control systems record all access attempts, who went through particular doors, and whether access was granted or denied, and may be used to generate a list for individuals, doors, groups such as attendants, passengers, cleaning, and maintenance crews. Higher security for access control in critical areas can be obtained with fingerprint readers or other biometric devices. No matter what type of identification is used, the basic issue of access control is limiting access to secured areas to authorized personnel. When access control is violated, an alarm will go off.

Perimeter security is defined as protection of a facility or area from external intruders (or internal escapees). A typical home burglar alarm system is a perimeter security system designed to protect the entire home using door alarms and a motion detector to sound an alarm when an intruder enters the home. For an airport, perimeter security is slightly more complicated because the airport runways may be the only area of concern. Chain-link fence with barbed wire may surround and protect the exterior of the airport aprons, taxiways, and runways. However, access control within the secured terminal area faces dual objectives of security and safety. The fact that anyone can open exit doors provides the possibility that someone may gain access to the runways. Gated loading doors and exit doors inside the terminal need to be easy to open, large enough to accommodate wheelchairs, and numerous so that everybody can easily exit the terminal in the event of a fire. These exit doors and the fire alarm doors may be alarmed with or without loud sirens. Yet, gated loading doors can not be controlled easily because keys, personal identification numbers, or ID cards can be borrowed or easily copied. The hard truth is that perimeter security alarms in a home or business actually can do little to stop a threatening offender from entering an area.

One unintended consequence of access control systems is false alarms that sound when users fail to properly close doors, enter using incorrect procedures, or tamper with a system so that it sends a false signal. User error is the most common cause of false alarms in commercial and home environments (Alarm Industry Research and Educational Foundation 1999). False home burglar alarms are a problem because police may be responding to a false burglar alarm while a crime is occurring elsewhere.

Alarms are intended to deter a potential offender from choosing a target. A key question about the benefits of alarms is whether a potential offender will choose another target because of an alarm (displacement) or choose not to commit a crime (diffusion). Displacement may mean different things such as an offender choosing another target (bank or home) because it does not have an alarm; an offender using another tactic to enter a location, for example, going in a window because the door is alarmed; or the offender coming back at a different time when the alarm is not on. Research as a whole seems to suggest that alarms lead to diffusion. Nonetheless, the debate over displacement versus diffusion remains complicated because studies use imperfect definitions in setting geographical boundaries and they use time limits for evaluation that are too short (Bowers and Johnson 2003).

Hakim, Rengert, and Shachmurove (1995) suggest that home burglar and fire alarms provide a net benefit to society. The household with false alarms does cost the community because there is an expense involved anytime police respond to an incident, yet the home alarms provide a social benefit in terms of fewer rapes, assaults, and burglaries and reduced fire damage due to earlier detection. Quite clearly, the hijackings of September 11, 2001, show that the costs of a single security breach in an airport can be substantial for the incident, consumer confidence, and lost business.

Graham Farrell (1995) argues that crime prevention policy should advocate the use of rapid response alarms. His work in Great Britain finds that people (and places) who are victimized have very high likelihoods of revictimization. Farrell makes a convincing argument that quick response alarms may deter revictimization and that they may be used to efficiently locate offenders, which prevents additional crime.

By installing and managing an alarm system to first prevent unauthorized access to secured areas, deterrence, detection, and response may follow. The stronger the security system is, the more it will act as a deterrent, and the need to respond to (false positive) security breaches is reduced. Improving access control through enhanced technology and training will reduce false system alarms and ultimately enhance crime prevention.

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