When are roadways usually the most slippery after rain, snow, or sleet?

Roadways are typically the most slippery right after it starts to rain, snow, or sleet. This is primarily due to the combination of moisture and the oil that has accumulated on the road surface over time. When it first begins to rain, the water mixes with these oils to create a slick layer on the pavement, significantly reducing traction for vehicles.

After a prolonged period of rain, the oils are often washed away, and while the surface can still be wet, it is generally less slippery than during the initial moments of rain. Similarly, once rain has stopped, the roadway might have puddles or standing water, but the initial slipperiness is usually resolved. Finally, sunlight can help to evaporate moisture and dry out the roads, improving traction.