Road tar is made from a mixture of hydrocarbons produced by processing crude oil or natural gas. Bitumen is the remnant of crude oil after the gasoline, kerosene, diesel oil and other fuels have been removed.
Bitumen is solid at low temperatures, but turns into a liquid at high temperatures. Tar is usually applied while hot so that it flows down into road cracks easily. Once cooled it turns into a semisolid substance. On days with a high ambient temperature tar becomes soft, and can become loose and pull away from the roads. This leads to the oil contained inside the tar being sprayed onto the lower panels of vehicles passing over them.