We've written before about casual carpooling, also known as slugging, where commuters are picked up by drivers looking to avoid toll charges by filling their cars with other passengers.
This article claims that over 20% of commuter traffic over the Bay Bridge are casual carpool vehicles. Given the amount of traffic in the San Francisco Bay Area, this figure is mind boggling, especially since it is a completely grass-roots phenomenom without any funding or oversight.
A key feature is a toll charge that drivers can avoid by being a carpool. There is no toll on the eastward section of the Bay Bridge and hence no casual carpooling.