You’ve probably seen road signs advising that bridges freeze before roads, but do you know why? Having open air underneath the bridge means the cold air surrounds the bridge both above and below.
If there’s even the chance that a bridge might be frozen, SLOW DOWN! And do it before you cross the bridge — changing speed on ice is dangerous.
There are two main reasons why bridges freeze before roads, even when the air temperature seems the same:
1. Exposure to the elements:
- Bridges lack insulation: Unlike roads, which are built on the ground and retain some heat from the earth below, bridges have nothing beneath them to shield them from the cold air. This means they lose heat from both the top and bottom, causing them to cool down much faster than roads, especially on windy nights.
- Materials conduct heat quickly: Most bridges are made of steel or concrete, which are excellent conductors of heat. This means any heat the bridge traps quickly transfers to the surface, where it’s lost to the surrounding air. Roads, on the other hand, are often made of asphalt or concrete, which are not as good at conducting heat, so they retain heat for longer.
2. Wind and moisture:
- Increased wind exposure: Bridges are often more exposed to wind than roads, especially those located in open areas or over bodies of water. This windchill effect further accelerates the heat loss from the bridge’s surface, making it freeze even faster.
- Moisture accumulation: Bridges are more prone to collecting moisture from fog, rain, or sleet due to their exposed nature. This moisture can freeze on the cold surface much faster than on the road, creating a layer of black ice that’s particularly dangerous for drivers.
Remember: Even if the road appears clear, be extra cautious when approaching bridges in cold weather, as they may be icy and hazardous. Slow down, increase your following distance, and be prepared for sudden changes in traction.