The Golden State is famous for its traffic jams, and access to HOV lanes is a particularly prized perquisite. A legacy vehicle must have a minimum of either two or three occupants to drive in an HOV lane, but the various types of advanced-fuel vehicles may use the lanes even with a single occupant – as long as the appropriate Clean Air Vehicle sticker is displayed.
The rules governing HOV lanes are complex, but for EV and PHEV drivers, things are actually quite simple: white stickers are for pure battery electric vehicles, and green stickers are for plug-in hybrids. If Governor Jerry Brown signs the new bills into law as expected, the privileges pursuant to both types of stickers will be prolonged until January 1, 2019.
Source: Green Car Congress