On an evening in August 2007, Minneapolis commuters sat in rush-hour traffic on the I-35 highway bridge that spanned the Mississippi River. Some drivers probably glanced at the construction crew resurfacing the concrete deck of the eight-lane, steel truss structure. Suddenly, a terrible wrenching noise sliced through the summer night. People screamed and honked their horns. A section of the bridge plummeted 60 feet straight down into the river, and the rest of the structure crumpled, sending 50 cars sliding into the water.

The collapse, which resulted in 13 deaths and 145 injuries, pointed to the importance of repairing aging infrastructure. As a result, Minnesota’s Department of Transportation (MnDOT) looked into new ways to conduct their biennial inspections on the state’s 12,961 bridges that carry traffic (830 of which urgently need repairs). Recently, drones replaced workers at several inspection sites, allowing the agency to get a closer look at the structures without closing a lane of traffic and sending a worker over the edge.

The eight lane bridge spanning the Mississippi River near Minneapolis’s downtown was undergoing repair work when it collapsed during the evening rush hour.

MnDOT uses the senseFly eXOM drone, a four-pound machine resembling a bumblebee that was specially built for mapping and inspection. Hooked up with an LED light and a camera, the drone can illumine dark spaces and capture detailed pictures. Oftentimes, the images have better resolution than inspectors can snap with a digital camera, while either perched atop a cherrypicker or suspended by rope underneath a bridge, says Jennifer Zink, a state bridge inspection engineer. A drone’s flight controller can toggle with an infrared camera, giving heat-sensing capabilities to pick up on distressed spots in the concrete.

The technology not only gathers better data, it also keeps workers safe. Even when traffic lanes are closed, a surprising number of drivers head into a work zone, swerving away from disaster at the last minute. The drone, on the other hand, steers clear of any objects, automatically bouncing away when it detects something closer than one to five feet.

The site of the collapse, Blatnik Bridge in Duluth, Minn., now uses drones to inspect the infrastructure.

Zink says the MnDOT team has been excited to innovate with new technology, but the law still lags behind, restricting when and where they can fly. There’s a lot of bridges in the state, Zink says, but with drones, managing their safety doesn’t seem like such a high-flying task.

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