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Emergency Evacuation

The recent report titled Emergency Evacuation Report Card 2006 evaluates the 37 largest U.S. urban areas to identify the evacuation challenges facing planners and residents in each area. The report uses a computed “Evacuation Index” to score, grade and rank each urban area based on automobile accessibility, internal traffic congestion and the highway capacity of major exit routes. The report finds that 25 of the 37 “could face greater challenges than New Orleans experienced after Hurricane Katrina.”
Tucson was not one of the 37 urban areas included in the report. However, the evacuation index for Tucson has been computed by Pima Association of Governments (PAG) using the report’s methodology, resulting in the following scores, grades and rankings: 

  • With the highest and best evacuation index score of 90.0, Kansas City is ranked No. 1 of the 38 urban areas, which earns it a grade of “A”
  • New Orleans has an evacuation index score of 67.3, which earns it a grade of “D” and rank of No. 12 
  • Tucson has an evacuation index score of 63.5, which earns it a grade of “D” and rank of No. 16
  • Phoenix has an evacuation index score of 43.6, which earns it a grade of “F” and rank of No. 31

The methodology used for the Emergency Evacuation Report Card 2006 is relatively simplistic. And it can be argued that Tucson’s score, grade and rank should be worse than computed above because the report’s methodology does not take into account the distribution of Tucson’s population relative to the location and orientation of the Tucson’s major exit routes, and the physical constraints associated with getting Tucson’s population onto the major exit routes. Even so, the results of this study are useful because they highlight the very real issues and challenges facing many U.S. cities, including Tucson, in planning and preparing for a mass evacuation. Like the 25 urban areas identified in the report, Tucson too could face greater challenges than New Orleans experienced after Hurricane Katrina.
The following approach and data was used to compute the evacuation index for Tucson:

Automobile Accessibility (AA) is an estimate of the percentage of the population that would be evacuated by car. Based upon the Katrina experience, it is assumed that all households that own a car (which includes 91 percent of Tucson households) will evacuate by car. In addition, it is assumed that one half of the households without a car (4.5 percent of Tucson households) will evacuate with relatives and friends who do have a car. Tucson’s Auto Accessibility score is 91 percent + 4.5 percent, or 95.5 percent (an “A”). In comparison, New Orleans’ score is 91 percent (an “A”) and Phoenix’s score is 96.4 percent (also an “A”).

Internal Traffic Flow (ITF) is an estimate of the ability of the roadway system within the urban area to deliver traffic to the exit routes. The Texas Transportation Institute’s “travel time index,” a nationally recognized indicator of traffic congestion in U.S. urban areas, is used to measure and score internal traffic congestion for this study. Using this measure, Tucson has a score of 69 out of 100 (a “D”), compared to New Orleans’ score of 81 (a “B”) and Phoenix’s score of 65 (a “D”).

Exit Capacity (EC) is a measure of the ability of major routes leading out of an urban area to accommodate the evacuating population. The standard for evaluation is 50% of the theoretical capacity of the roadways at the exit points over a 12-hour period, assuming use of contra-flow on the exit routes. Using this measure, New Orleans scored 67 out of 100 (a “D”), while Phoenix scored 25 (an “F”). The comparable score for Tucson is 64 (a “D”), which assumes  I-10 EB, I-10 WB, I-19 SB, SR-77 NB and SR-86 WB can all be used as exit routes. However, Tucson’s score could be as low as 16 (an “F”) if, for example, only I-10 EB could be used as the primary exit route.

Evacuation Index (EI) is used to score and rank each urban area, and is computed by averaging the Internal Traffic Flow (ITF) score and the Exit Capacity (EC) score, and factoring the result by the Auto Accessibility (AA) score: EI = ((ITF +EC) / 2) * AA. Tucson’s computed evacuation index score is 63.5. 


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