California's HOV lane sticker program -- which is meant to give drivers incentive to adopt more efficient, but more expensive, electrified vehicles -- was scheduled to come to a close at the end of 2015, but a bill approved by the state legislature has extended the program until 2019.
Drivers of fully electric vehicles (such as the Tesla Model S, Nissan Leaf, or Fiat 500e) and plug-in hybrid vehicles (such as the Toyota Prius Plug-in or Chevrolet Volt) can receive white or green stickers, respectively, that will allow them full access to high-occupancy vehicle carpool lanes, even if they're driving solo.
Drivers of non-plug-in hybrids will no doubt remember and lament that the HOV lane sticker program was modified in 2011 to remove access for standard hybrids, which used yellow HOV lane stickers.
HOV carpool lane access, the shorter commute times that come along with it, and a number of federal and state tax incentives and rebates work together to help drivers to justify the higher cost of entry of plug-in vehicles. Well, that and the feeling that you're saving the Earth one mile at a time.