Informal car pools – called “casual carpooling” in the San Francisco Area, and “slugging”* in the Washington, D.C. area, and also known as "ad hoc carpooling." In various locations people and vehicles queue in order to form car pools that take advantage of high-occupancy vehicle lanes that offer significant time savings, and – in the San Francisco case – toll savings. The systems are completely informal; there is no central or government oversight.
Washington, D.C.-area website: Slug-Lines.com
* At least two explanations are commonly offered for the origin of the term “slugging:” (1) riders are “slugs” because they slow, lazy (while drivers are “body snatchers” since the riders might otherwise have used public transportation); (2) “slugs” are fake coins that could be used in transit fareboxes (likewise reflecting the riders’ defection from public transportation).