When are roadways typically the most slippery?

The first rain after a dry spell is when roadways are typically the most slippery due to a combination of factors. When it has not rained for a while, oil and other substances accumulate on the road surface. As the rain begins to wash these substances away, it creates a slick and hazardous condition, especially in the early moments of rainfall when the water mixes with oil and debris, making the roads particularly dangerous for vehicles. This initial period of rain can often catch drivers off guard, as they may underestimate the slippery conditions.

In contrast, during a heavy downpour or after it has rained for a while, the rain may wash away much of the accumulated oil on the roadway, leading to less slippery conditions than right after the dry spell has been broken. The other options do not account for the specific combination of oil, debris, and rain that creates the most treacherous driving conditions immediately after a long dry period.