Bridges freeze before roads due to being exposed from above and below, being over cold spots, and their construction. Ice can be hidden under snow on bridges, so drive carefully in cold weather and assume bridges are frozen. Use chains or snow tires and avoid hard braking or sharp turns on icy roads.
Most people living in cold regions have seen signs saying “bridge freezes before road” and many people are aware that care should be taken when driving over bridges in cold weather as they can freeze easily. Bridges freeze before roads for a variety of reasons, many of which involve basic science. Being aware of the conditions that cause bridges to freeze will make you a better driver, and if you live in a cold region of the world, make sure that newcomers to your area are aware of the danger of bridges during cold or icy weather .
There are three main reasons bridges freeze before roads. The first is that a bridge is exposed from below and above, unlike a roadway. Roads don’t freeze over as quickly because they can take advantage of the insulation and warmth of the ground below. Bridges, on the other hand, are beset by cold temperatures from all sides, which can cause them to freeze over quickly. Bridges also freeze quickly because many of them are over cold spots, such as frozen rivers or deep ravines.
The third reason bridges freeze before roads is their construction. The materials used in the core of a bridge are excellent conductors of heat. As a result, they extract heat from within the bridge, transferring it to the surface where it is rapidly lost when ambient temperatures are very low. Bridges often heat up to general air temperature quickly, so when the thermometer reads 32 degrees Fahrenheit (0 degrees Celsius), the bridge freezes over.
You should be careful when driving over bridges in freezing weather, even if you see no ice on the bridge. Sometimes, ice can hide under patches of snow, causing you to lose control. This is especially true in areas that they don’t regularly climb, sand, or plow; an accumulation of ice and snow may have settled on the bridge, making it very dangerous. Also, ice can sometimes be hard to see on dark roads, especially at night when ambient temperatures get even colder.
Because bridges freeze before roads, you should assume bridges are frozen when the ambient temperature is very cold, even if the roads are clear. Always drive carefully in cold weather and use chains or snow tires in regions where they are recommended. It’s also a good idea to check the tread wear on a regular basis and you should change your tires if they wear out, as this can be very dangerous on icy roads. If you should lose control of your car on an icy patch, take your foot off the accelerator and concentrate on steering out of the skid; do not brake hard or turn the wheel sharply, as this may cause fishtailing, skidding or overturning.
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