Police behaving predictably: The other enemy

     At 3 p.m. on a Saturday in June 2007, an SUV loaded with propane canisters was deliberately driven into the glass entrance doors of the main airport terminal at Scotland's Glasgow International Airport and set ablaze. The canisters did not...


     At 3 p.m. on a Saturday in June 2007, an SUV loaded with propane canisters was deliberately driven into the glass entrance doors of the main airport terminal at Scotland's Glasgow International Airport and set ablaze. The canisters did not detonate, and security bollards outside the entrance prevented the vehicle from entering the terminal.

     The incident, however, reveals the relative ease with which explosive devices can penetrate airport security schemes, illustrating that absolute protection of strategic national infrastructure components like airports is impossible.

     Vehicle traffic, entrance roads and perimeter fences cannot be monitored at all times due to limited police and security resources, leaving airports largely vulnerable. Dallas-Fort Worth Airport sprawls over 28 square miles. Denver International Airport (52 square miles) is twice the size of Manhattan.

     This lurking security exposure is aggravated by the fact that terrorists can study security arrangements that already exist, and exploit any predictable patterns to their advantage. Terrorism is only one enemy; predictability is the other.

     "Randomizing schedules for patrolling, vehicle checking and explosive monitoring is thus an important tool in the police arsenal to avoid the vulnerability that results from predictability," says Milind Tambe, a professor of computer science at the University of Southern California (USC).

     Tambe was instrumental in the emergence of a new security system in use at Los Angeles International Airport. The program, called Assistant for Randomized Monitoring Over Routes, or ARMOR, is controlled by a sophisticated computer system based on game theory.

     Game theory is a branch of applied mathematics that attempts to mathematically capture behavior in strategic situations, in which one individual's success in making choices depends on the choices of others. Game theory, generally credited to mathematician John von Neumann in 1944, and later refined by mathematician John Forbes Nash in 1950, was initially developed to analyze competitions in which one individual does better at another's expense. It has subsequently been expanded to treat a range of scientific and social interactions.

     In this particular game, police attempt to preemptively counter terrorist strategy by anticipating future behavior based on the analysis of past behavior, while at the same time randomizing police behavior to lessen or eliminate predictability of deployments and operations.

     "The ARMOR system provides an intelligent approach for randomized patrolling, monitoring or vehicle checking," says Tambe, who is with the Center for Risk and Economic Analysis of Terrorism Events (CREATE), a Department of Homeland Security (DHS) Center of Excellence based at USC. CREATE works with government agencies and other researchers to evaluate the risks, costs and consequences of terrorism. The center helps policy-makers set priorities and find the best, most efficient ways to counter threats and prevent attacks.

     The ARMOR system does not merely toss dice to determine a random schedule. That might generate randomization, but would not guarantee, quality of the security provided.

     "The key to ARMOR is its game theory foundation, which provides intelligent weights in randomization," Tambe says. "That means the dice are intelligently loaded, yielding results that allow quality guarantees on the protection provided."

How it works

     ARMOR considers the actions of law enforcement agencies against adversaries as a strategic game. Law enforcement agencies act first in this game - creating security arrangements, setting up defense systems and securing vulnerable areas. Adversaries have a second-mover advantage and can observe these arrangements to plan an attack. Terrorists get to observe patrols, shifts and techniques while attempting to discover predictable patterns.

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