When is the road most likely to be slippery?

The road is most likely to be slippery when it has just begun to rain due to the initial combination of rain and oil residue that has accumulated on the roadway. This oil can create a slick surface when mixed with water, leading to reduced traction for vehicles. When rain starts to fall, it can lift and mix with the oil, making the surface particularly hazardous for drivers until enough rain has fallen to wash this oil away.

In contrast, heavy snowfall typically results in more obvious surface hazards, but the slipperiness may not be as pronounced immediately unless the snow has started to melt or compact into ice. Extreme heat can cause pavement to soften, leading to other issues, but it is not directly related to slipperiness like the onset of rain. A long dry spell may lead to dusty roads and could affect visibility and traction, but it does not create the immediate slippery conditions seen at the beginning of rain.