The impact of coastal flooding on traffic resilience: The case of San Francisco Bay Area

A new study suggests that coastal flooding and seawater level rise severely impact the traffic network and safety in San Francisco Bay Area, USA.

Seal level rise in California will potentially become a severe problem in the following decades. In fact, the water level is expected to rise by more than 2 meters by 2100 and flooding could become a daily issue for numerous people. In financial terms, about $8-$10 billion of infrastructure could be underwater within the next 30 years and an additional $8-$10 billion worth of property will be at risk of tides.

Traffic resilience is practically the ability of a transport system to withstand congestion. Coastal floods and high seawater levels are becoming more frequent in regions with relatively low altitudes due to the impact of climate change. At the same time, many coastal cities experience a population boost and accumulate numerous people and resources. Coastal floods can result in three types of impact: 1. Completely blocked roads where commuting is totally restricted, 2. Delays where some roadways remain functional and travelers need to use reroutes and 3. Increase of road and pedestrian accidents as a result of high vehicle flows in local roads.

San Francisco Bay is a highly populated region that has a congested road network. Nowadays, minor coastal flood incidents cause disproportional traffic problems that 1. increase the travel times and 2. result in more accidents.

In such a stressed system, disruptions can readily take place when a natural disaster (such as a flood) strikes. The impact of those disruptions can be direct or indirect. The direct consequences refer to the damage caused to roads and infrastructure whilst indirect damage is associated with travel delays, increased accidents and employee absences. The new study focuses on the indirect impacts of seawater rise and flood events to evaluate the latter. 

The findings of the study suggest that, during coastal flooding, travel time delays increase in areas where the road network does not offer alternative routes to follow while accidents rise in regions that have a well-developed secondary network and traffic flows quickly shift. In addition, accidents are more pronounced in low water level rise while more delays were detected in high water levels. The authors suggest that, given the findings, accidents are more of a threat than delays at "low-to-moderate water levels".

In addition, flooding of major highways sends commuters to local roads in residential areas nearby. The neighborhoods around highways are historically less developed in terms of infrastructure, thus, road safety is further compromised.