When are roadways typically the most slippery?

Roadways are typically the most slippery during the first rain after a period of dryness due to the accumulation of oils, dirt, and debris that settle on the pavement. When rain begins to fall after an extended dry spell, it can create a film that makes the surface very slick, particularly for the initial moments of the rain. This phenomenon is caused by the water mixing with these substances, reducing the traction between tires and the road.

At the first onset of rain, the water does not immediately wash away these contaminants, and the slippery conditions can lead to an increased risk of accidents. As the rainfall continues, the water gradually cleans the road surface, making it less slippery over time.

In contrast, heavy rainfall can often lead to wet but not necessarily slippery conditions once the road has been rinsed, newly paved roads can have a rough surface to improve traction, and freezing temperatures lead to icy conditions that also create hazards, but the initial slipperiness is particularly pronounced following a dry spell.