Why is the surface most slippery at the onset of rain?

The surface is most slippery at the onset of rain primarily because oil and dirt that have accumulated on the road are mixed with the water. This combination creates a very slick layer on the road surface, significantly reducing traction for vehicles. When it starts to rain, the initial moisture lifts these materials off the road, forming a slippery film. This is particularly hazardous for motorcyclists and drivers, as it can lead to loss of control.

Other factors, such as the rubber on tires or the condition of road signs, don't contribute as significantly to the slipperiness during the initial rain. While water does have an effect on tires, the critical aspect in this scenario is the mixing of oil and dirt with rainwater. In essence, the slickness is a direct result of the contaminants densely packed on the roadway, which becomes problematic when disturbed by rainfall.