Tolls increased July 1, 2010 on all Bay Area bridges except the Golden Gate Bridge, including carpool and some vanpool tolls. The discounted toll is only available to 3-person carpools and hybrids with FasTrak in designated carpool lane(s).
http://rideshare.511.org/511maps/tolls.aspx
FasTrak Carpools get discounted bridge tolls during peak commute hours with FasTrak and access to carpool lanes. (Carpools consists of 3 or more people per vehicle)