What causes white foam on roads after rain?

White foam on roads after rain is often caused by a combination of pollutants, such as soap, detergents, and oils, mixing with rainwater. When rainwater washes these contaminants off surfaces and into storm drains, agitation from rain or water flow can create foam. This phenomenon is more pronounced in urban areas where runoff carries higher concentrations of chemicals. Additionally, organic matter and algae can contribute to the foamy appearance in certain conditions.