When are roads most slippery?

Roads are most slippery during the first rain after a dry spell because the rain mixes with the accumulated oil and debris that has built up on the road surface. During dry weather, oil can seep up from the pavement and accumulate, creating a slick layer. When rain first falls, it does not immediately wash this layer away; instead, it can make the road surface exceptionally slick and hazardous. Drivers may find it challenging to maintain traction, increasing the risk of skidding or losing control of the vehicle. Over time, as rain continues, the oil is washed away, and traction improves.

In contrast, heavy fog does not affect road surface traction in the same way, and while the initial moments of rain can be slippery, the most hazardous condition occurs right after the rain begins following a prolonged dry period. After snowfall, roads can indeed be slippery, but the danger typically arises from ice or packed snow rather than the initial wet conditions created by rain.