When is rain covered-road most slippery?

The first few minutes of the rain are the most dangerous. The rain causes the oils and contaminants in the road to lift to the surface. You are not only driving on water, but water mixed with these oils.

well... when covered by a film of liquid sufficient enough to reduce the coefficient ecstatic friction between the tyre and the road to essentially zero, but not so deep as to introduce a new source of friction