Flood-related Accidents in Suburbia: The Far-reaching Impacts of Coastal Flooding on Non-Highway Car Accidents in the San Francisco Bay Area

Abstract:

A rising global sea level and intensifying coastal flood events increase the risks of disruption of traffic networks in low-lying, coastal communities. The consequences of flood-related traffic disruption go far beyond the areas of inundation and affect the entire region. The inundation of roads and major traffic corridors forces commuters onto alternate routes, thus aggravating congestion and causing spillover traffic onto local roads that pass through urban and suburban communities. The increases in traffic volumes in urban and suburban communities can lead to a spike in traffic accidents and cause loss of life.

In this study, we build on our previous work of quantifying travel time delays and employee absences associated with flood-related traffic disruption in the San Francisco Bay Area. We quantify the increases in non-highway car accident rates by simulating morning traffic patterns in the region under flooding conditions. We partition the region into zones and develop quadratic regression models that relate simulated traffic volumes to historical accident rates within each zone. We use an incremental traffic assignment model to simulate traffic patterns during morning hours. To identify inundated roads, we use a set of regional flood maps that represent extreme water levels resulting from combinations of storm surges, tides, oceans swells, and sea level rise. We derive hourly car accident rates from the California Statewide Integrated Traffic Records System, and use the regression models to project accident rates under flooding conditions. By restricting to non-highway accidents we can estimate the impacts of flood-related traffic disruption on urban and suburban residential communities.

Our study shows that the increase in accident rates is spatially extensive, reaching far beyond the areas of inundation, especially in the North Bay region. The increases are associated with the inundation of key nexus points of traffic, causing region-wide changes in traffic patterns and leading to spillover traffic in urban and suburban regions. Our analysis highlights the multifaceted nature of flood-related traffic disruption and speaks to the need for a holistic and regionally coordinated adaptation plan for coastal flooding and sea level rise.