When are highways typically most slippery?

Highways are typically most slippery right after it starts to rain following a dry spell due to the accumulation of oil and grime on the road surface. When rain begins, it mixes with these substances and creates a slick layer, making the road particularly hazardous. This is especially true if there has been a significant period without precipitation, as the oils can build up and make the roads dangerously slick right when the rain begins to wash them away.

During heavy rain, while roads can become slippery, the initial oils and grime may already be washed away, leading to less slipperiness compared to the first moments of rain. Low temperatures can contribute to slippery conditions, particularly with icy roads, but they are not the primary concern in the context of initial rain. Similarly, after snow melts, roads can be wet but are usually not as slick as immediately after the onset of rain following a dry period because the snow typically does not leave the same oily residue.