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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