When do bridges typically freeze during cold weather?

Bridges typically freeze when first wet due to several factors that affect their temperature and exposure. Unlike regular road surfaces, bridges are elevated structures with air circulating both above and below them. This means that they lose heat more quickly than road surfaces that are insulated by the ground. When a bridge becomes wet and temperatures drop, the moisture can freeze rapidly, making the surface slippery and hazardous.

The other scenarios mentioned, such as bridges freezing when first built, after being shaded by trees, or only during heavy snowfall, do not fully capture the mechanics of how and when freezing occurs on bridges. A newly constructed bridge might not immediately present freezing issues unless it becomes wet in cold temperatures. Being shaded can impact freezing but is more circumstantial and doesn't universally apply. Heavy snowfall can contribute to ice formation, but it can be deceiving, as freezing can occur with little moisture if temperatures are sufficiently low. Therefore, the most reliable condition that leads to freezing is when bridges become wet and the temperature drops.