The Miller-McCune site describes how commuters in the Washington DC area have developed an informal people's mass transit system known as "slugging" to take advantage of the HOV-3 lanes leading out of the city.
Their culture exists almost nowhere else. San Francisco has a similar casual-carpooling system, and there’s a small one in Houston. But that’s it. Even in D.C., slugging exists along only one of the city’s many arteries, I-95 and 395, where the nation’s first HOV lanes were completed in 1975.
Every morning, these commuters meet in park-and-ride lots along the interstate in northern Virginia. They then ride, often in silence, without exchanging so much as first names, obeying rules of etiquette but having no formal organization. No money changes hands, although the motive is hardly altruistic. Each person benefits in pursuit of a selfish goal: For the passenger, it’s a free ride; for the driver, a pass to the HOV lane, and both get a faster trip than they would otherwise. Even society reaps rewards, as thousands of cars come off the highway.