What surface condition poses the greatest hazard during the early stages of rain?

The slippery, oily film on the road poses the greatest hazard during the early stages of rain primarily because rainwater initially mixes with dirt, oil, and other residues that accumulate on the roadway, creating a very slick surface. This combination results in a dangerous condition, especially for vehicles, as the increased slipperiness can lead to loss of traction.

During the first moments of rain, this oily layer can dramatically reduce friction between the tires and the road, potentially causing skidding or loss of control. As the rain continues, this film is usually washed away to some extent, which can improve traction.

Wet leaves and potholes filled with water can also be hazardous, but they don't present the same immediate and widespread risk that the oily film does when the rain first starts. Increased visibility issues are a concern, but they don’t directly affect vehicle traction in the same way that a slippery road surface does. Overall, the oily film is a particularly treacherous condition that drivers must be cautious of when rain begins to fall.