Flexible carpooling is carpooling that is not arranged ahead of time, but instead makes use of designated meeting places. It seeks to replicate the informal "slug-lines" that form in Washington DC, Houston, and San Francisco, by establishing more formal locations for travelers to form carpools without advance contact. It is estimated that San Francisco saves almost 1 million gallons of gasoline and associated emissions each year because of the operation of the informal flexible carpooling system.