The pavement on bridges can be icy even when other pavement is not. This is because bridges can be colder and more icy than other roadways. When the temperature is around the freezing point, ice can become wet. This makes it more slippery than at temperatures well below freezing. If it starts to rain on a hot day, pavement can be very slippery for the first few minutes.
Heat causes the oil in the asphalt to come to the surface.
The road is more slippery until the oil is washed off. When it is raining or the road is wet, most tires have good traction up to about 35 mph. However as you go faster, your tires will start to ride up on the water, like water skis. This is called hydroplaning. In a heavy rain, your tires can lose all traction with the road at about 50 mph.