The most insidious type of road icing threat comes from bridges and overpasses. Those signs you always see ARE true. A bridge is exposed to air on all of its surfaces - on top, underneath and on its sides. By contrast, a normal road surface is only exposed to air on one side, its top surface. When temperatures drop, this means bridges will cool and accumulate snow and ice faster than roadways on solid ground.

An icy bridge's most dangerous threat is their element of surprise - they catch drivers off guard, who are travelling at full speed because the rest of the roads are either clear or just a little wet. The consequences of driving onto ice at highway speeds can be catastrophic, as the loss of control and impacts happen much faster than in most other conditions. Slides are often unrecoverable and chain-reaction type accidents are common, as additional vehicles will often lose control in the exact same location.

Notice the bridge above, completely covered with a layer of very slick snow and ice, even though the roads before the bridges are totally clear. The transition from clear road to icy takes place in mere inches! To get a visual understanding of this idea, view the animation file linked below.