Roads are most slippery right after it begins to rain because:
The water mixes with oil and grease on the road.
The roads are naturally slippery.
The gravel gets washed to the middle of the lane.
Some tires are not designed for rain.
Roads are particularly slippery when it first begins to rain. The water mixes with oil drippings on the road that have not yet washed away, creating a slippery and dangerous mixture.