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A study comparing evacuation patterns in response to two 2024 hurricanes, Milton and Helene, found that people in coastal areas with frequent hurricane exposure were much more likely to travel out of harm's way compared to people in inland areas who were more likely to stay put.

Researchers at Columbia University Mailman School of Public Health led the study. Their findings in the journal Environmental Research Letters.

These geographic differences are likely due to a combination of factors, including access to transportation infrastructure, social norms, and risk perception, the authors argue.

On September 26, 2024, Helene (Category 4 at both its strongest and landfall) made landfall near Perry, Florida, before moving inland, where it led to historic rainfall and flooding in Georgia, South Carolina, North Carolina, Tennessee, and Virginia, resulting in an estimated 219 deaths and losses of $78.7 billion. Thirteen days later, on October 9, Milton (Category 5 at its strongest and Category 3 at landfall) made landfall near Siesta Key, Florida, and passed through central Florida, resulting in an estimated 32 deaths and losses of $34.3 billion.

The researchers compared aggregated cell phone data on human mobility in the affected regions. While there were disruptions to cell phone service during the hurricane periods, the researchers focused on relative changes in mobility, which were largely unaffected by outages.

Milton primarily impacted coastal areas with frequent hurricane exposure, prompting sharp increases in out-of-region travel before landfall and sustained elevated mobility post-disaster. In contrast, Helene primarily affected inland areas, where mobility changes were relatively modest; affected coastal counties showed stronger mobility responses than inland counties.

Hurricane Milton led to a significant rise in out-of-region (29%) movement beginning three days before landfall. In contrast, during Hurricane Helene, out-of-region movement increased only modestly (5%) in the three days before landfall, despite emergency declarations and evacuation orders.

Access to financial resources and transportation infrastructure may shape people's decisions and ability to evacuate, the authors argue. Areas with higher socioeconomic status and population density in coastal regions may have greater access to transportation, information, and resources, enabling faster evacuation. In contrast, more dispersed inland areas with lower levels of socioeconomic status are likely to lack these supports, potentially limiting their mobility.

Climate change extends the reach of hurricanes into inland areas

There has been significant attention given to the way that climate change is increasing the frequency and destructiveness of hurricanes. Climate change is also pushing hurricanes into new areas and presenting new risks not often associated with hurricanes. Most fatalities from Hurricane Helene occurred in counties with low historical hurricane risk, including many counties where the researchers saw low levels of evacuation.

In inland areas, most deaths were due to river flooding, infrastructure damage, and indirect causes such as power outages and medical emergencies. In coastal communities, storm surges were the primary driver of fatalities.

"Social norms, past experiences, and the way people perceive their risk all shape how people respond to threats like hurricanes," says study first author Qing Yao, Ph.D., an associate research scientist in environmental health sciences at Columbia Mailman School. "People in coastal areas are used to the idea of evacuating, whereas people living in inland areas may be more likely to think they are protected."

"As climate change expands the geographical range of hurricanes into previously unaffected areas, there is an increased need for tailoring disaster preparedness and response strategies to the affected populations and the risks they face," concludes senior author Sen Pei, Ph.D., assistant professor of environmental health sciences at Columbia Mailman School.

Additional authors include Victoria Lynch, Xiao Wu, and Robbie Parks at Columbia Mailman School, and Molei Liu at Peking University, Beijing, China