Why does the highway become slippery after rain?

The roads are most slippery during and after the first rain after a long time without rain. Many vehicles are leaking some kind of liquid, usually oil, from their vehicles. They remain on the road. Tires break and leave residue on the sidewalk. Oil from asphalt sidewalks can even wash out over time as the asphalt deteriorates. Asphalt emulsion usually contains soapy material. Dust and debris can also be on the road surface. The first rain will wash these materials away and the road may be slightly or very slippery.