Engineering-Based Hurricane Risk Estimates and Comparison to Perceived Risks in Storm-Prone Areas

Shadow evacuees have negative effects on hurricane-induced evacuation operations. The goal of this study is to understand and explain shadow evacuation by comparing perceived risks with engineering risk estimates at the household level—particularly in nonevacuation zones where undesirable evacuation decisions are made—rather than by comparing perceived risks with regional hazards that previous studies have measured. A proposed methodology to compute engineering risk estimates from hurricane hazards is illustrated for approximately one million properties in Harris County, Texas. These risks are then compared with risk perceptions from a survey of residents which was conducted after Hurricane Ike’s landfall in 2008. The results show that evacuation behavior is related to a hurricane-induced surge and wind risk mismatch and that risk overestimation and underestimation are more likely seen in locations near the waterways. The results also show that shadow evacuation is partly attributable to risk overestimation. The overestimation of damage risk from wind or surge correlates to 108,031 shadow evacuees. This is more than the total number of households in Galveston County that need to evacuate in the event of a hurricane—primarily through Harris County—and who may be subjected to traffic delays caused by shadow evacuees. The spatial distribution of risk mismatch provides opportunity for emergency managers to effectively plan and respond to hurricanes, while preventing unwanted evacuation.