When do roads typically become very slippery during a rain storm?

Roads typically become very slippery during the first 10 to 15 minutes of a rainstorm because this is when oil and debris on the road surface mix with the rainwater. Initially, the rain does not fully wash away these contaminants, and the combination creates a very slick condition. As the rain continues, the water gradually washes away the oil and dirt, reducing the slipperiness of the road. Therefore, the critical early moments of rainfall, when there's a new mix of rain and substances already on the road, are especially hazardous for drivers.