Flooding of highways, bridges, tunnels, and other roadways interrupts daily life throughout the U.S., delaying or blocking passage of emergency response vehicles and people trying to get to work or school. These flood events stall local economies and put public safety at risk. In Maryland—which is home to diverse geography and communities, from the Appalachian Mountains and farmland to large cities and thriving beach towns—much of that flooding occurs outside designated flood zones, according to a study released today.
“Flooding Impacts on Maryland’s Transportation System and Users,” a report by strategic consulting firm ICF with support from The Pew Charitable Trusts, examines how flooding affects nearly 15,000 lane miles of state-maintained roadways—about 20% of lane mileage in Maryland—and the drivers who use them. The findings draw from data provided by the Maryland Department of Transportation (MDOT) for the years 2006 to 2020.
The researchers reviewed 2,771 flood-related incidents for which geospatial data was available and found that 78% occurred outside the 100- or 500-year flood zones mapped by the Federal Emergency Management Agency (FEMA). The incidents took place, on average, only about 0.3 miles from the mapped flood areas, but even so, the findings underscore that flooding is not limited to mapped zones. And in fact, FEMA’s maps are not intended to predict the full extent of likely flooding, but rather to identify places requiring flood insurance, show areas at relatively high risk of major flooding, and help state and local decision-makers manage threats.
The report also identifies locations along state highways that are especially flood-prone, making them prime targets for infrastructure resilience or relocation investment. Data from the past 15 years shows clusters of flood incidents on state highways, including more than 100 locations with at least five flood events within about 1,000 feet of one another. Seven locations appear to be especially at risk, with at least 30 such incidents among them.
The report also shows how flooding can disrupt travel and cause safety risks, productivity loss, and other adverse consequences. Flooding of state-maintained roadways in Maryland accounts for weeks of traffic disruptions annually, averaging 1,582 hours—or 66 days—a year. And although most lane closures that caused disruptions lasted less than four hours, 16% of all disruptions lasted longer than 12 hours.
These incidents affected, on average, more than 480,000 people annually. The disruptions can also be measured economically. User delays captured in this report—which include the value of lost work time and delayed deliveries—cost about $15 million per year in Maryland and totaled more than $230 million during the study period. Each flood incident resulted in an average of about $80,000 in user delay costs. And that is just a fraction of the fiscal impact because it does not factor in other expenses, such as emergency response and infrastructure repairs.
Although this report focuses on the frequency and cost of flooded roadways, Maryland is nevertheless a leader in scientifically evaluating coastal risks to address repeated flooding. For example, MDOT has used climate data to assess the vulnerability of more than 8,500 structures to sea level rise, storm surge, and precipitation change. MDOT has also reviewed the exposure of the state’s roads and compiled an index that shows their risk to storm events. Further, since 2015, Maryland’s innovative Coast Smart program has developed important policies, such as the 2020 Coast Smart Construction Program guidelines, which outline design criteria for state-funded capital projects that improve climate resilience.
Nevertheless, flooding can hinder livelihoods throughout communities in any year, even without a major event. And as events become more frequent and intense—slowing or halting traffic, inundating roads, and damaging infrastructure—all levels of government must provide greater support to Maryland and other states to more accurately map risk, pass evidence-based building standards, and ensure that transportation systems are flood ready. These proactive steps can help lower the number of incidents and reduce costs.
Matthew Fuchs works on The Pew Charitable Trusts’ flood-prepared communities project.