Almost half of the world’s population currently lives in cities, and that number is expected to increase significantly in the near future. This rapid urbanization adds to the increased risk of flooding due to the growing concentration of people and resources in cities and the accumulation of cities along the coasts.

These urban shifts are also leading to more complex and interconnected systems that people depend on, such as transport networks. Disruption of urban transport networks from floods or other natural disasters can have serious socio-economic consequences. Indeed, the indirect effects of these types of events, such as employee commute-related absences, travel time delays, and increased traffic accident rates, could ultimately outweigh the more direct physical damage to roads and infrastructure from severe flooding.

Stanford researchers examined transportation networks in the San Francisco Bay Area (SF Bay Area) as a case study to quantify the indirect effects of sea level rise and intensification of coastal flood events on urban systems. Specifically, researchers sought to determine the impact of flooding on traffic delays and safety, particularly as road closures divert vehicles into adjacent streets and residential areas that were not designed to handle heavy vehicle flows. The study was published in the May issue of Urban Climate.

“The goal is to highlight road safety in future climate adaptation planning,” said lead study author Indraneel Kasmalkar, a graduate student in engineering at the Stanford Institute for Computational and Mathematical Engineering (ICME).

Similar to many other regions of the country, the SF Bay Area has dense urban development along its coast and heavily congested transport networks. Currently, even relatively minor coastal floods have the potential to flood large corridors and increase the already long commute times and traffic accidents.

“I think one of the main problems with traffic in the bay is that we’re already pretty close to the limit,” said senior study author Jenny Suckale, assistant professor of geophysics at Stanford University’s School of Earth, Energy & Environmental Sciences. Earth). “That’s why some of the relatively low water levels we’re considering here can make a big difference.”

Three types of flood impacts have been identified for coastal flood events: impassable commuting routes where the origin, destination or critical road links are flooded and impede driving; Travel time delays caused by commuters rerouting to avoid flooded roads; and increases in car and pedestrian accident rates in high-traffic communities as commuters divert to local roads.

The study highlights the challenges in preparing the Bay Area’s transport network for climate change. Increasing coastal flooding could result in significant delays in travel time across the Bay Area, including communities that are not themselves affected by flooding. However, focusing solely on reducing travel time delays can be problematic, as coastal flooding will affect some communities mainly through increases in accident rates.

The research follows the team’s recent results, published in Science Advances, which showed that commuters living outside of the floodplain areas may experience some of the largest commuting delays in the Bay Area due to the nature of the road network in the area.

“The two studies provide interesting contrasts on the resilience of communities to flood exposure,” said Kasmalkar.

While delays increase sharply at higher water levels, accident rates increase most across the region at low water levels, suggesting that accidents may be a bigger problem than delays at low to medium water levels. Using the travel time lag metric alone to estimate traffic resilience could target travel efficiency rather than road safety in the planning effort.

When freeway floods force commuters onto local roads that pass through residential areas, there is an increase in the number of accidents. This can particularly affect lower-income or historically disadvantaged communities that are more likely to be on highways and may have fewer road safety regulations.

“Some communities may care more about safety than traffic delays,” Suckale said. “Networking makes governance and decision-making difficult, and planners don’t necessarily take into account the negative consequences for the neighbors.”

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Suckale is also kindly a Fellow of the Stanford Woods Institute for the Environment, Assistant Professor of Civil and Environmental Engineering, and a member of the ICME.

The research was supported by the UPS Endowment Fund for Transportation, Logistics and Urban Issues, Stanford’s Bill Lane Center for the American West, NSF through the Office of Polar Programs, and Stanford University. The work is the product of the Stanford Future Bay Initiative, a research, education and practice partnership committed to co-producing actionable information with communities in the San Francisco Bay Area to build a fairer, more resilient and sustainable urban future.

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