Clean Air stickers, available in green for plug-in hybrids and white for CNG or battery-electric vehicles, allow drivers to use California’s High Occupancy Vehicle (HOV) lanes without having two or more passengers.

Introduced in February 2012, the scheme was originally due to issue a limited run of 40,000 plug-in hybrid stickers but has been very popular with consumers. The California Air Resources Board ran out before the start of May, but the State will now issue an additional 15,000 stickers to the first applicants.

The two-tier system replaced a similar scheme for hybrids, which expired in 2011. This offered a powerful incentive for drivers, allowing vehicles which could drive solely using an electric motor to access the HOV lanes provided they had a yellow bumper sticker, and was said to cut the average commuting time by a third – or 36 minutes.

Its replacement is proving similarly popular, with 40% of Chevrolet Volts sold in California alone, and recent legislation extended its original 30th September 2017 expiry date to the 1st January 2019.

Extending the scheme to 55,000 green stickers should continue to fuel growth of the plug-in hybrid sector, which includes the BMW i3 REX, Cadillac ELR, Ford Fusion and C-MAX Energi and Toyota Prius Plug-in, ahead of a replacement which is likely to include incentives for hydrogen fuel cells.