Why is the road surface often slippery during the first half hour after it starts to rain?

The correct answer emphasizes that during the initial period of rain, the road surface can become especially slippery due to the combination of water mixing with oil, dust, and other substances that have built up on the pavement. When rain first begins, it doesn't immediately wash away these contaminants. Instead, the water can cause the oils to rise to the surface, creating a slimy layer that reduces traction for vehicles. This mixture is particularly problematic because drivers may not anticipate the reduced grip, leading to an increased risk of skidding or loss of control.

The other factors, while they can contribute to slippery conditions, do not specifically address the reason for the heightened slipperiness immediately after the rain begins. Wet leaves, for instance, can create hazards but are not as significant as the oil and dirt combination. A loss of traction can occur but is more a consequence rather than a direct cause in this specific scenario. A significant drop in surface temperature may affect grip, but it is the combination of the contaminants and water that primarily leads to the immediate slipperiness.