The existing yellow stickers expired on Friday, with 85,000 letters sent out to registered vehicle owners over the last two months. Solo drivers in a hybrid vehicle using a High Occupancy lane will be subject to the same fines as those in conventionally powered cars.

Introduced in 2004, the yellow sticker programme allowed carpool exemption to drivers of vehicles with partial zero emissions and 45mpg or greater highway fuel economy. Stricter standards will be introduced for the new generation of Clean Air stickers, due to be introduced in 2012.

The DMV estimates that the new standards will allow 40,000 vehicles to use the carpool lanes, but specific types have yet to be identified. California's white sticker programme, which applies to pure electric vehicles, is valid until 2015.