Originally reserved for vehicles with two or more occupants, in 2004 California’s High Occupancy Vehicle (HOV) lanes were opened up to single-occupant vehicles capable of running on electricity alone and with 45mpg or greater fuel economy provided they applied for a yellow bumper sticker.

Credited with driving mass adoption of hybrids in the State, it’s a powerful incentive for drivers. According to the DMV, access to the 1,400 miles of HOV lanes cuts 36 minutes, or up to a third, of the average Californian’s commuting time.

The original yellow sticker scheme expired in July 2011, removing HOV lane access for hybrids, but was replaced in February 2012 to reflect new drivetrain technology. This introduced green stickers for the first 40,000 plug-in hybrid drivers, valid until 2019, while owners of plug-in hybrid and CNG vehicles were offered an unlimited number of white stickers until the January 2015 cut-off date.

But figures posted on the California Air Resources Board have shown the scheme is running low on green stickers. As of the 7th April 2014, 38,478 have been issued to plug-in hybrids in the State, with eligible vehicles including the Chevrolet Volt, Ford Fusion Energi and Toyota Prius Plug-in. Chevrolet said in a statement last year that 40% of Volts sold in the United States are in California.

The Board has yet to announce whether the scheme will be renewed, however further schemes are likely to encourage uptake of hydrogen fuel cell vehicles. Toyota and Honda have both indicated that California will be a focus for the launch of the new technology.