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Global optimization of emergency evacuation assignments.

Article Excerpt
Conventional emergency evacuation plans often assign evacuees to fixed routes or destinations based mainly on geographic proximity. Such approaches can be inefficient if the roads are congested, blocked, or otherwise dangerous because of the emergency. By not constraining evacuees to prespecified destinations, a one-destination evacuation approach provides flexibility in the optimization process. We present a framework for the simultaneous optimization of evacuation-traffic distribution and assignment. Based on the one-destination evacuation concept, we can obtain the optimal destination and route assignment by solving a one-destination traffic-assignment problem on a modified network representation. In a county-wide, large-scale evacuation case study, the one-destination model yields substantial improvement over the conventional approach, with the overall evacuation time reduced by more than 60 percent. More importantly, emergency planners can easily implement this framework by instructing evacuees to go to destinations that the one-destination optimization process selects.

Key words: emergency planning; microscopic simulation; traffic assignment; network optimization; mass evacuation; special event operations; dynamic traffic assignment; intelligent transportation system. History: This paper was refereed.

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Heightened interest in emergency management Land evacuation operations has impelled transportation professionals to focus on evacuation modeling and planning. Most evacuation-modeling research has used conventional transportation-planning models, such as the four-step process, as a basis. The design of these planning models assumes their use in day-to-day travel under normal situations where origins and destinations of trips are easy to determine and, for practical purposes, remain somewhat static over time. Emergency evacuation, on the other hand, calls for expeditiously mobilizing and transporting a sizable population out of danger, overcoming temporal and spatial constraints in situations where they might encounter roads that are congested, blocked, or otherwise dangerous because of the emergency. Although the transient, and potentially chaotic, nature of evacuation makes it far more challenging to manage, planning an evacuation has a unique advantage over planning normal traffic operations. Under normal conditions, a motorist takes a specific route to arrive at a specific destination; that motorist is more flexible under emergency-evacuation conditions. In other words, as long as an evacuee safely exits the evacuation zone quickly, the route taken is not important. Emergency planners do not clearly understand this flexibility in destination selection and its associated benefit to the planning process; therefore, they seldom exploit it. In fact, the common practice of rigidly assigning evacuees to designated routes, shelters, or destinations based mainly on proximity often negates this flexibility.

Experience suggests that a major problem in evacuation operations is that evacuation-zone exit routes are often limited in number and insufficient in capacity to handle the traffic surge during a large-scale emergency evacuation (Urbina and Wolshon 2003). In most cases, constructing new routes and increasing roadway capacities are simply too costly. Therefore, studies have often focused on methods to improve the planning and operational aspects of the evacuation process to maximize the utility of the existing transportation network. Studies have included the implementation of counter and contra-flow lanes (Han and Franzese 2001), staggered departure times, traffic-signal control, multiple-jurisdiction coordination, and special routing consideration for heavy vehicles (Han et al. 2005). However, no study recognized or explored flexibility in evacuee destination selection until Yuan and Han (2005) proposed the concept of "most desirable destination" for evacuees. This concept recognizes that municipalities responsible for large-scale evacuation have routinely assigned evacuees to routes and destinations based on limited experience and intuition rather than methodical optimization processes. The origin-destination table used in most evacuation models represents a static assignment that rarely leads to optimal efficiency because traffic demand and roadway conditions fluctuate and shift throughout the evacuation period. Even with the implementation of dynamic-traffic assignment, a technique that considers changing traffic conditions to find the best evacuation routes, models that are based on fixed origin-destination tables are inefficient when a destination becomes difficult (or impossible) to access due to congestion or blockage. In this study, we explore options that allow evacuees flexibility in selecting their exit routes and destinations.

Destination assignment and route assignment...