When is the road surface most slippery?

The correct choice, when it first starts raining, is based on the fact that the initial moments of rain can lead to the most slippery road conditions. When rain first begins to fall, it interacts with oil, dirt, and debris that have accumulated on the road surface. This combination creates a slick film that reduces traction. As the rain continues, this layer may be washed away, and while the road can still be slippery, it generally becomes less hazardous as the rain continues.

In contrast, during heavy rain, although the road can be slippery, the excess water tends to wash away some of the oils and debris, which can reduce slipperiness over time. After it rains, the road may still be wet and slick, but the conditions are not as hazardous as during the first rainfall. During winter storms, icy conditions can occur, but the specific question was about the slipperiness caused by rain rather than snow or ice.