We should pay special attention to bridges and overpasses when driving in temperatures around freezing point. We’ve learnt that bridges may be icy while the road isn’t, and that’s all we need to know to for safety reasons. But, giving it a second thought, how could the bridge be icy when the road isn’t? Here’s why.
You’re driving in light drizzle. The road surface is wet and the road grip is just like on an ordinary rainy day. You encounter a long bridge, and all of a sudden, the road is covered with pure ice. If you noticed that temperature is a bit below freezing you’re hopefully prepared for this situation, but if you thought temperature is so high that it rains instead of snowing, then you may have a very unpleasant – and dangerous – surprise…
The bridge is surrounded by cold air
Even though temperature is below freezing it still can rain (if you want to know why, read the blog post Icy road basics for safe driving). And also, if it snows, the snow can melt if the road is warmer than the surrounding temperature. In both these situations the road surface will be wet. Even while the road surface temperature is dropping, the heat underneath the road keeps it warm enough to prevent icing - the road traps heat.
Now, let’s look at the bridge. Also the bridge loses heat to the surrounding air, but since the freezing wind strikes the thin bridge from all sides, it’s chilled much faster than the road. There is no way the bridge can trap heat, so the bridge temperature will drop fast and the bridge will freeze shortly after surrounding air temperature hits the freezing point.
This is the reason why bridges are icy while the roads aren’t. But, there is also a material feature that further increases the difference between the road and the bridge.
Heat conductivity vs. insulation
Different materials have different ability to conduct heat. If you want to conserve heat, like in a house, you should use building materials that have low heat conductivity. This improves insulation and reduces heat loss. Steel is a good heat conductor, i.e. it easily “transfers” its heat to the surroundings, while wood is not. Actually, that’s why steel feels colder than a piece of wood, despite they’re having exactly the same temperature. Also, concrete is a good heat conductor, while asphalt is not.
The bridge is mainly constructed of steel and concrete, which contributes to a faster temperature loss due to high heat conductivity. The road, on the other hand, is covered with low heat conductive asphalt, which further contributes to conserve heat in the road.
For safe driving, always be extra attentive to bridges and overpasses in temperatures around freezing. Chances are that road grip is completely different on the bridge!
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