A bridge deck can freeze solid while the road on either side remains completely clear because, in contrast to an open roadway, cold air moves both above and below it. For many years, the only effective defence was a salt-filled truck that was dispatched anytime circumstances were dangerous and frequently arrived after ice had already formed. After deciding that responding to ice was insufficient, Minnesota's transport experts set out to build a bridge that could heal itself before a single tyre lost traction. The experiment took place on the Interstate 35W bridge in Minneapolis, which crosses the Mississippi River. Due to significant year-round traffic and mist rising from adjacent St. Anthony Falls, the area is prone to black ice. According to Minnesota's Department of Transportation, the state installed its first automated bridge anti-icing system on the old Interstate 35W bridge over the Mississippi River in 1998.
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Building a bridge that senses its own weather
The system itself was a Swiss-designed installation supplied by a company called Boschung, and it worked by connecting weather sensors to a network of spray nozzles placed directly into the road surface. The system connected weather sensors to spray nozzles embedded in the bridge deck, while its computer-controlled system could select from 16 different spray patterns depending on storm conditions.
Instead of regular salt, the device sprays a chemical called potassium acetate when conditions get hazardous since it works at significantly lower temperatures and is less corrosive to the understructure. Next to the bridge was a tank containing several thousand gallons of the chemical, which could be pumped away automatically the instant sensors detected the possibility of ice forming, without the need for maintenance personnel to be sent out.
What the numbers actually showed
The system was subsequently evaluated using operational and crash data, and a federal review reported a 68% reduction in winter-season crashes at the I-35W Mississippi River bridge. The federal evaluation reported a 68% reduction in winter-season crashes at the I-35W Mississippi River bridge, with a benefit-to-cost ratio of 3.4 to 1. The same analysis also found that, depending on the location, automatic anti-icing devices on bridges generally reduced crashes by 25–100%.
Ice control becomes commonplace
Minnesota later deployed automated anti-icing systems at other bridges, including sites near Duluth, Winona, and elsewhere in the state. Since then, independent research organisations have compiled a list of several similar projects around the nation, viewing the original Mississippi River bridge as a sort of proof of concept for the entire technology, a case still referenced in transport research archives maintained by the Transportation Research Board . The Minneapolis installation showed that automated bridge anti-icing could reduce winter crashes while treating the road before ice formed. Minnesota subsequently used automated systems at other high-risk bridge locations across the state.
