Follow the Leader Syndrome: Motorists’ Response to Flash Flooding in Texas

Flash floods pose specific risks to the public due to their quick-onset characteristics which limit warning lead time. Texas leads the nation in flash flood fatalities, of which almost 80% are vehicle related. Previous studies have shown people enter floodwater due to poor risk perception, social influences, demographics, roadway familiarity and environmental factors. In order to address the increasing vulnerability and exposure to flash flood threats, the Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) collaborated with local stakeholders to conduct direct observations of motorists' behavior and surveys of public perceptions around flood waters in the Dallas Fort Worth Metroplex (DFW).

The following research questions were addressed: 1) How do motorists respond to High Water Warning System flashers during flash flood events?; and 2) What are the different influences and characteristics of those who drive through flooded waters, “crossers” and those who take another route “diverters?” Using a mixed-methods approach, first, motorists were recorded using a webcam positioned at a low-water crossing in Fort Worth, Texas during two back to back flash flood events. Motorists’ decisions to divert or proceed through the flashers were recorded. Second, surveys were deployed to residents of DFW using the neighborhood app Nextdoor and the CASA Alerts app immediately following a flash flood event. Respondents were asked about sources of weather information, vehicle type, if flooding was observed, and if so, where, what did they do, and what factors influenced that decision.

Results from the webcam study reveal a majority of motorists ignore the flashers and proceed through the low water crossing. Motorists were largely influenced to either divert or cross based on “following the leader” or watching the behavior of motorists in front of them. Environmental factors, such as water depth, as well as congestion, also play a role in diversion rates. Survey results also revealed environmental cues, such as perceived water level and speed, rainfall intensity and duration were “very” or “extremely” influential for motorists to divert, as were social cues, such as seeing other vehicles stuck or the presence of barricades, flashers, or officials. Survey results also suggest the weather information and warnings received by the public during an event lack geographically specific information for flash flood threats.

This study was conducted in the context of the CASA WX Living Lab for Severe Weather Warning and Response in partnership with the City of Fort Worth Departments of Stormwater Management, Emergency Management and Community Engagement, with support from the North Central Texas Council of Governments. Combined, these studies point to consistent findings that can inform stakeholders on how motorists perceive and respond to flash flooding events, and what influences motorists in their decision-making. A flood warning strategy is complicated and should have the potential to be tailored to individuals needs and perceptions, and to be geographically specific.