Excerpt from Wikipedia Article on Slugging:

Slugging, also known as casual carpooling, is the practice of forming ad hoc, informal carpools for purposes of commuting, essentially a variation of ride-share commuting and hitchhiking. While the practice is most common and most publicized in the congested Washington, D.C. area (where it is primarily used by commuters who live in Northern Virginia), slugging is also used in San Francisco,Pittsburgh, and other U.S. cities. Sluggers gather at local businesses and at government-run locations, albeit not always with official sanction.