When is the road surface most slippery?

The road surface is most slippery immediately after rain begins due to the initial mixing of rainwater with oil and other substances that accumulate on the road surface. During dry periods, oil, grease, and other residues can build up on the pavement. When rain starts, these substances become slick and can create a hazardous driving situation until the rain adequately washes them away. This is particularly true for the first few minutes of rain, as the water has not yet had time to thoroughly cleanse the road, making it critical for drivers to exercise caution during this period.

In contrast, after several hours of rain, the road surface typically becomes less slippery as continuous rainfall helps to wash away the oils and contaminants. Similarly, during winter snowfall, ice formation poses a danger, but it is a different concern than the slipperiness caused by rain on a dirty surface. Hot summer days may lead to different challenges such as heat-related performance issues for vehicles, but they do not typically affect road slipperiness in the context of rain.