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tion routes for up to 18 h, resulting in vehicles running out of gasand obstructing the normal flow of traffic. The evacuation systembecame gridlocked and frustrated evacuees. Should Rita have beena direct hit on the Houston area, the ramifications would have beenunthinkable.

In response to the Rita evacuation experience, the Houston–Galveston Area Council (H-GAC), together with other agenciesresponsible for the safe and adequate transportation flow in themetropolitan area, set out to identify bottlenecks in the transportsystem and policies that could more effectively move evacueesduring the next natural disaster. With the goal of improving under-standing of evacuation transport system operations and impacts ontravelers, the H-GAC identified the need for a tool that could be usedto test policies and operational decisions that could results in safe,secure, and efficient evacuations.

This paper describes the development of such a tool, specificallya hurricane evacuation model for the Greater Houston area, as a prod-uct of the combined efforts of H-GAC, the Texas TransportationInstitute (TTI), and Citilabs. A dynamic traffic assignment (DTA)model, incorporating mesoscale simulation of queues on evacua-tion routes, was integrated into the H-GAC regional travel demandmodel system and used to analyze a base scenario representing theRita event. Controls were then added providing capabilities to test theeffects of both demand- and supply-side scenarios upon the referenceevacuation outcome. This model is intended for screening and eval-uation of evacuation plans based upon systemwide performance andzone-specific clearance times.

A secondary purpose of this paper is to describe some of the tech-niques applied in this project found to be useful in working with thisvery large dynamic network loading and mesosimulation problem, inthe hope of assisting others embarking upon similar tasks. Althoughthis was not the first evacuation model to be developed by means ofWorld Airport Transportation Statistics (WATS), nor perhaps thefirst WATS model applied at such a large scale, the project wasnonetheless perhaps unique in the practicality of its aims and thepragmatism of the approach ultimately selected.

BACKGROUND

Evacuation planning is a logical and appropriate context for applica-tion of WATS because, unlike normal weekday conditions, evacuationconditions are characterized by extremely concentrated time-varyingorigin–destination (O-D) demands resulting in highly unstable anddisorderly link flows with oversaturated queues. Barrett et al. (1), inproposing a WATS-based emergency traffic management system,outline the reasons why traditional planning models fail to provide

Development of a Strategic HurricaneEvacuation–Dynamic Traffic AssignmentModel for the Houston, Texas, Region

Colby Brown, Wade White, Chris van Slyke, and Jimmie D. Benson

This paper describes a hurricane evacuation model for the GreaterHouston, Texas, area whose base case reproduces the widespread con-gestion and gridlock experienced during the evacuation response toHurricane Rita in 2005. The project—a result of the combined effortsof the Houston–Galveston Area Council (H-GAC), the Texas Trans-portation Institute (TTI), and Citilabs—incorporated the developmentof a dynamic traffic assignment model for evaluating the performance ofmajor evacuation routes within the H-GAC region. To minimize modelrun time while maximizing policy analysis capabilities, a strategic modelwas developed on the basis of the full H-GAC regional model networkand zone system. This strategic model integrates tightly with the regionalplanning model and can be updated with new information concerningthe structure of background demand derived from normal weekdaytrip tables. Evacuation trip tables were developed by TTI based uponsurvey data collected shortly after the 2005 event. In the networkassignment model, background and evacuation trips are incrementallyloaded by using a dynamic, generalized cost framework that can bemodified to account for route preference, traveler information, trans-portation supply dynamics, and congestion feedback over a 72-h modelperiod. Policy analysis tools provided with the model include supply-sidecontrols such as contra-flow lane reversal timing, facility closures,incident response, as well as demand-side controls such as evacuationtrip departure scheduling. The model can be applied to comparison ofthe relative system and evacuation corridor performance of alternativepolicy scenarios.

In September 2005, the greater Houston, Texas, area was threatenedby Hurricane Rita. In the weeks before Rita’s threat, New Orleans,Louisiana, was devastated by Hurricane Katrina. The recent NewOrleans experience was a stark reminder of the power of hurricanesas witnessed in the Galveston, Texas, destruction early in the 20th cen-tury. Houston and Galveston residents took the Rita threat seriouslyand evacuated the metropolitan area in record numbers.

One by-product of the large-scale evacuation was unprecedentedtraffic congestion. Many travelers were stuck on the major evacua-

C. Brown, Citilabs, Inc., 1040 Marina Village Parkway, Alameda, CA 94501. W. White, Whitehouse Group, 1901 McGuckian Avenue, Suite 121, AnnapolisMD 21403. C. van Slyke, Houston–Galveston Area Council, P.O. Box 22777,Houston, TX 77227. J. D. Benson, Travel Forecasting Program, Texas Transporta-tion Institute, Texas A&M University System, 1106 Clayton Lane, Suite 300E,Austin, TX 78723. Corresponding author: C. Brown, cbrown@citilabs.com.

Transportation Research Record: Journal of the Transportation Research Board,No. 2137, Transportation Research Board of the National Academies, Washington,D.C., 2009, pp. 46–53.DOI: 10.3141/2137-06