California Gov. Arnold Schwarzenegger signed Assembly Bill 1500 Wednesday, providing a perk that allows owners of pure electric vehicles and cars running on compressed natural gas to drive solo in California carpool lanes until Jan. 1, 2015.

Clean Air HOV Access Sticker

That’s good news for owners of the all-electric Tesla Roadster and Toyota Rav4 EV, and motorists planning to buy the all-electric Nissan LEAF—as well as the few who drive the Honda GX, which runs on compressed natural gas. But it’s bad news for the 85,000 or so drivers of the Toyota Prius, Honda Insight or Honda Civic Hybrid, who currently enjoy solo access to the diamond lanes.

If the governor signs SB 535, a second bill that is now working its way through the state legislature, those drivers of conventional hybrids could enjoy the carpool lane privilege beyond the current deadline of Jan. 1, 2011. But the perk would only last for an additional six months, until July 31, 2011. SB 353 would also offer the incentive to 40,000 owners of plug-in hybrids, such as the Chevy Volt, Fisker Karma, and the plug-in version of the Toyota Prius, which goes on sale in 2012. At this point, plug-in hybrids are also excluded from solo HOV lane access.

Although the law only applies to California, other states with solo HOV provisions—including Colorado, Utah, Florida, New Jersey, New York, Virginia, Tennessee, and Utah—could follow suit. California officials essentially will be removing 85,000 hybrids from the carpool lanes, to make room for the next generation of electric cars.

While solo driving in the carpool lane is highly coveted by some commuters, its effectiveness in promoting the growth of greener cars, and thereby reducing emissions, is uncertain. The state opened up carpool lanes to hybrids in 2004, but stopped issuing the yellow access stickers in early 2007. Yet, sales of hybrid cars continued to grow throughout 2007 and early 2008—largely spurred by rising gas prices. Since that time, hybrid sales have cooled with the downturn of the economy and the onset of lower gas prices.

Federal tax incentives for fuel-efficient cars have similarly shifted to electric vehicles—forcing conventional gas-electric hybrids to compete on an even playing ground with the rest of the automotive market. Some green car advocates question if it makes sense to remove tax credits and HOV perks from conventional fuel-efficient hybrids, which still represent less than 3 percent of the new car market.