How to Save America’s Crumbling Bridges

The weather in Minneapolis on August 1, 2007, was hot, topping out at 92 degrees. It was the perfect day to visit a water park, which is where a school bus carrying summer campers was returning from during the evening rush hour, when the I-35W bridge across the Mississippi River gave way. Thankfully, the bus didn’t plunge into the river—it got wedged between a guardrail and a burning semi-trailer—and all 63 kids made it out safely. But 13 others who happened to be driving over the bridge at the time of its collapse never made it home.

The I-35W disaster might have inspired federal lawmakers to invest more money in America’s 600,000-plus bridges, a quarter of which have been deemed either “structurally deficient” or “functionally obsolete.” But Congress can barely keep the highway trust fund solvent, let alone agree on a new transportation financing mechanism. Each time the funding question gets postponed, the average age of U.S. bridges goes up (it already exceeds 40 years), as does the risk of another disaster on the scale of Minneapolis’.

So what’s a local transportation department to do? Most cities and states that want to maintain safe bridges have essentially two choices: patch up existing problems or build entirely new structures. Both come with significant costs and divert money from other areas of the transportation network. But technology has given rise to a new tool that doesn’t require gambling on the structural integrity of dated bridges or blowing budgets on repairs or construction: real-time data.

Rather than relying on annual inspections that may or may not detect structural flaws, many states and cities have discovered that infrastructure sensors and the data they constantly collect can provide ongoing assessments of the lifespan of major bridges. These data not only help local authorities detect potential structural problems but also provide new ways to manage the flow of traffic over them. The result is a much more precise system for knowing when and how to invest in critical bridge maintenance—preventing both catastrophic failures and unnecessary replacements.

Take New York, where time, weather, and deferred maintenance have not been kind to many of the city’s East River crossings. The Brooklyn Bridge, an engineering marvel of its time, shows its age through the cracks in the masonry vaults that support the bridge’s roadway over Manhattan. Fiber-optic sensors monitor these cracks, as well as other indicators such as temperature fluctuation, to assist structural engineers in determining when the vaults will ultimately need to be replaced. Further up the East River, on the Williamsburg Bridge, a series of interferometric and fiber Bragg grating sensors (both capable of measuring light waves) monitor wire deformation and breakage on the span’s century-old suspension cables. Rather than make an annual manual inspection, engineers have access to continuous data, which can tell them if an individual strand in one of the bridge’s cable is about to break.

In Minneapolis, one of the triggers of the I-35W collapse was the large number of construction vehicles parked on the bridge, which ultimately helped buckle the bridge’s gusset plates. Well-placed sensors could have detected this weight increase and the resulting stress it placed on critical structural elements of the bridge, potentially allowing the bridge to be closed before it collapsed. Local officials packed the I-35W replacement bridge with hundreds of sensors to monitor climate, traffic, and structural fluctuations—including 195 vibrating wire strain gauges. These sensors have taken much of the guess work from the visual bridge inspection process.

This revolution in data cuts both ways. Some bridges are over-engineered and can last longer than highway planners originally anticipated. Data from sensors installed on bridge girders can tell local officials if a bridge is carrying loads that exceed its capacity. In many cases, the sensor data shows that bridges can remain in service for longer than inspectors thought or that weight restrictions are unnecessary. Replacing a perfectly good bridge with years left before the end of their natural lifespans is just as foolish and wasteful as allowing an unsafe bridge to remain in service longer than necessary.

Data can change the way we use bridges, too, and in the process help cities and states prolong their functional lives. E-Z Passes and automatic tolling can reduce or end long toll booth lines, signal data can help us coordinate lights to accommodate traffic flows on streets and bridges, and pricing technology can charge drivers for road use according to demand. (The latter can also help pay for bridge design, building, and maintenance—especially if Congress lifts the ban on tolling existing interstate highways.) By smoothing traffic peaks, these tools can prevent overuse during intense rush hour periods and help keep bridges in working condition—ultimately squeezing more life out of existing infrastructure.

Local governments have already started exploring these options. The Evergreen Point Bridge, a congested span in metropolitan Seattle, uses variable priced tolls to spread out traffic flows. In Washington, D.C., the District Department of Transportation is examining whether to turn part of the notoriously congested 14th Street Bridge into a high occupancy-express toll bridge. With a new crossing over the Potomac a near political impossibility, data-driven tolling may be not only be the best option for easing traffic on one of the most important arteries in the nation’s capital, but also provide a source of much-needed revenue for maintenance.

As things stand, Americans must accept that Congress and the states simply aren’t in the fiscal position to undertake the wholesale rebuilding of our nation’s bridge infrastructure. That means the bridges we already have are going to be used more efficiently. By harnessing data, and coupling technology advances with public-private partnerships, transportation agencies in cities and states across the country can help make our infrastructure last longer while policymakers try to find consensus on how to fund the country’s roads and bridges for the next half century.

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Stephen Goldsmith is the Daniel Paul Professor of the Practice of Government and the Director of the Innovations in Government Program at the Harvard Kennedy School. His latest book is The Responsive City.