When are roads and highways considered most slippery?

Roads and highways are considered most slippery immediately after it starts to rain following a dry spell due to the accumulation of oils and debris on the roadway surface. When rain first begins to fall, it mixes with these oils and creates a slick surface that can be particularly hazardous for drivers. This effect is especially pronounced after a long dry period, as the oils have more time to build up.

In contrast, during heavy rainfall, the water can wash away some of the oils, thus reducing slipperiness. While winter months can present slippery conditions due to ice and snow, the immediate rainfall scenario presents a unique hazard primarily rooted in the interaction of water with accumulated surface oils. On hot days, the road may be dry, and while certain conditions can cause changes in traction, they do not typically create slippery conditions akin to the described scenario.