What causes roads to become particularly slippery at the start of rainfall?

When it begins to rain, the initial drops of water mix with oils and other pollutants that have accumulated on the road surface. This combination creates a slippery layer. The oils, typically from vehicles, rise to the surface when the pavement is dry and can be particularly hazardous when rain first begins, as they create a slick film that reduces traction for vehicles and can lead to skidding.

The presence of high humidity levels does not directly cause roads to become slippery, nor does pedestrian traffic have an impact on the slipperiness of wet roads. Although rain does wash away dirt and debris over time, it is this initial interaction between water and oil that primarily contributes to reduced traction at the start of rainfall.