We should pay special attention to bridges and overpasses when driving in temperatures around the freezing point. Over the years we have heard that bridges may be icy while the road is not. We have also seen signs posted before bridges and overpasses which state this for safety reasons. This condition however is an important one that should be given some consideration. So how could the bridge be icy when the road is not? 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 suddenly, the road is covered with pure ice. If you have noticed that the temperature is at freezing or just a bit below you’re hopefully prepared for this situation. If, however, you thought the temperature was above freezing enough that it rains instead of snows, then you may have a very unpleasant – and dangerous – surprise…
The bridge is surrounded by cold air
Even though temperatures are below freezing it still can rain (if you want to know why, read the blog post Icy road basics for safe driving). And, 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. In the case of the bridge it loses heat to the surrounding air, and since the freezing wind strikes the thin bridge from all sides, it is chilled much faster than the road. Since there is no way the bridge can trap heat; the bridge temperature will drop fast, and the bridge will freeze shortly after the surrounding air temperature hits the freezing point. It is for this reason that bridges are icy while the roads are not. 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. That is why steel feels colder than a piece of wood, even though they have 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.
When it comes to safe driving in near freezing conditions always be extra attentive to bridges and overpasses. Chances are that road grip is completely different than on the bridge! The following link provides more tips on what to think of when driving in winter weather.