When are roads most slippery during rain?

The reason why roads are most slippery when it first starts to rain is due to the oil and debris that accumulate on the road surface. When it hasn't rained for a while, oil from vehicles, dirt, and other substances build up on the asphalt. The initial rainfall causes this oily layer to rise to the surface, making the roads particularly slick and hazardous.

As the rain continues, the water dilutes the oil, and the road may become somewhat less slippery but can still remain wet. After a rainstorm has ended, the roads will generally dry out, reducing the slipperiness as well. Heavier downpours can create unsafe conditions due to flooding or standing water, but the immediate danger of slipperiness is most pronounced at the onset of rain. Thus, the most critical period for slippery roads is indeed right when it starts to rain.