Evacuation Planning

Events ranging from hurricanes to hazardous material spills represent environmental threats to people and property. Evacuation is the process of moving at-risk populations to safety by relocating them outside a defined threat area. As evacuation is primarily about human movement, it has substantial behavioral and transportation dimensions. Evacuation planning is the process of anticipating how to best move people to safety should a disaster occur. There are many problems that can arise in an evacuation that serve to delay or prevent officials from evacuating everyone quickly, and anticipating and ameliorating these problems is the principal concern of evacuation planning. The resulting evacuation plan describes aspects and contingencies in the evacuation process under the broad goal of moving people out of the threat area in the least amount of time and with the least amount of risk.

Evacuation is generally divided into six overlapping time phases: incident detection time, decision time, notification time, preparation time, travel time, and verification time. Incident detection time is the time it takes officials to detect that an event has occurred. In the case of a hurricane, this might be done via satellite days prior to landfall, but the challenge of detecting the release of an odorless, invisible toxic gas from a storage site would be much greater. Decision time is the time it takes officials to determine whether to recommend protective actions to the public. Notification time is the time it takes to communicate a warning message to the public through radio, television, the Internet, or other media. Preparation time is the time it takes the public to gather family members, pets, and valuables prior to departing. Travel time is the time it takes evacuees to move from an origin in the threat area to a safer destination. Verification time is the time it takes officials to determine whether everyone has left the threat area.

Each of these time phases can alternatively be viewed as a series of questions for evacuation planning purposes. The principal question in the incident detection phase is this: how will an event be detected and monitored in a timely manner? The answer to this question might be by remote (hurricane) or in situ sensors (tsunami), whereas in the case of a wildfire or sudden hazardous material (hazmat) highway spill, the plan might be to rely on eyewitnesses with cell phones. Three important questions that need to be answered in the decision-time phase include who needs to take action, what is the best action, and when do they need to take it?

The answers to these questions comprise the basics for an official warning message. The primary planning question in the preparation phase is how can one best prepare the public such that their response to a future evacuation recommendation is likely and timely? The travel-time phase is where the most significant transportation questions arise, including what are the likely origin-destination flows, travel modes, routing, and roadway control? The question in the verification phase is how will officials [Page 542]determine who has evacuated, who has not, and what should the at-risk population that is not clear of the threat area do (for example, shelter-in-refuge)?

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