When are roadways least slippery after it has been raining?

The least slippery conditions on roadways occur after it has been raining for a while. When rain first begins to fall, it mixes with oil, dirt, and debris that have accumulated on the road surface, creating a very slippery condition known as "black ice" or a slick layer. As the rain continues, this oil and debris are washed away, and the pavement becomes cleaner and less slippery.

While heavy rain can reduce slipperiness by washing away contaminants, it can also lead to other hazards like hydroplaning and reduce visibility. The initial moments of rain are particularly treacherous, as it takes time for the rain to effectively clean the road. After the first few minutes, traction improves significantly as the necessary cleaning process happens. Thus, the most favorable conditions for driving in wet weather are generally found after the rain has been ongoing for a sufficient period, making the roadway less hazardous.