Final answer:
Light rain mixes with oil and gasoline on the road surface, creating a slick layer that can reduce car control. Heavy rain can wash away some of the residue, potentially improving conditions slightly, but still poses risks like hydroplaning.
Step-by-step explanation:
When oil and gasoline leak onto the road surface from cars, it creates a layer that can make the road slippery. During a light rain, this mixture can rise to the top and create a very slick surface, reducing the traction a car's tires can get and therefore making it harder to control the car. Heavy rain can wash away more of the oily residues, which might provide slightly better conditions than during a light rain, but it also presents its own risks like hydroplaning due to water buildup between the tires and road surface.