Empirical Characterization of Mass Evacuation Traffic Flow

Wolshon, Brian · 2008 · Crossref

DOI: 10.3141/2041-05

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Summary

This paper addresses a critical gap in mass evacuation planning: the lack of empirical, field-based data on traffic flow characteristics during actual emergency evacuations. Historically, simulations and forecasts of roadway performance under evacuation conditions relied on conjecture and professional judgment rather than observational evidence. The study was motivated by the opportunity to analyze traffic data collected during the Hurricane Katrina evacuation in Louisiana in August 2005. The primary objectives were to characterize general traffic flow conditions, determine maximum sustainable flows, and compare these empirical findings to theoretical capacities outlined in the Highway Capacity Manual (HCM). Additionally, the study sought to evaluate the performance of contraflow operations (reversible lanes), a strategy previously debated due to a lack of quantitative performance data. The methodology relied on traffic volume data collected by the Louisiana Department of Transportation and Development (LA DOTD) from a network of 67 permanent automated count stations across the state. These stations covered various roadway functional classifications, including freeways, four-lane arterials, and two-lane arterials/collectors, in both urban and non-urbanized areas. The analysis focused on the evacuation period from August 27 to August 28, 2005, comparing these flows against baseline data from the preceding three weeks. The study specifically examined peak hourly volumes, duration of high volumes, and lane-specific flows, with particular attention to contraflow segments on Interstate 55 near Fluker, Louisiana. The findings revealed that during the Katrina evacuation, most roadways carried flows significantly below the maximum capacities predicted by the HCM. Contrary to expectations that enormous demand would saturate roadways, maximum flows on urban roadways typically did not even reach levels observed during typical daily commuter periods. For instance, peak evacuation flows on five of nine urbanized-area freeway segments were lower than non-emergency peaks. In non-urbanized areas, evacuation flows were higher than normal peaks, but still below theoretical maximums. Regarding contraflow, the study found that peak flows in contraflow lanes were approximately 30% lower than in adjacent normal-flow lanes, potentially due to cautious driver behavior or loading strategies. However, contraflow provided a significant net benefit, increasing total northbound volume on I-55 by 40% compared to the previous year’s Hurricane Ivan evacuation, where contraflow was not used. The evacuation process itself lasted approximately 36–38 hours, half the 72 hours previously estimated by officials. The significance of this work lies in providing the first substantial empirical dataset for evacuation traffic modeling. By demonstrating that actual evacuation flows are often lower than theoretical maximums and that contraflow, despite lower per-lane efficiency, substantially increases total system capacity, the study offers concrete parameters for future simulation models. These findings challenge assumptions that roadways will reach saturation during evacuations and provide evidence to support the implementation of contraflow strategies. The data aids planners in more accurately estimating clearance times and evaluating the utility of traffic management plans for future natural or man-made hazards.

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• traffic density