What condition makes the surface of a sealed road most slippery?

The condition that makes the surface of a sealed road most slippery is when it begins to rain. This occurs because, during the initial moments of rainfall, the rainwater can mix with oil and debris that has accumulated on the road surface. This combination creates a slick layer that can significantly reduce traction for vehicles. The first rain often brings these slippery materials to the surface, making the road particularly hazardous for drivers until the rain has washed them away and the surface has been properly absorbed by the road.

As rain continues and the surface becomes saturated, it generally becomes less slippery because the oils and contaminants are gradually washed away. In contrast, dry conditions or prolonged periods without rain can lead to a buildup of these slippery substances, but they tend to be less hazardous than the immediate effect of the very first rain.