California electric-car drivers have a new color to keep track of. Starting New Year's Day 2019, the state will issue updated Clean Air stickers that allow electric cars purchased after January 1 to access the state's carpool lanes, even without a carpool inside.

The new purple decals will remain valid through Jan. 1, 2023, a year longer than the red stickers issued in March that extended the eligibility of cars with earlier white (electric car) or green (plug-in hybrid) stickers. The red stickers expire Jan. 1, 2022. White and green stickers expire at the end of this year.

California purple carpool sticker

California purple carpool sticker

Of course, as with most regulations, there's a catch: The stickers are only available for new plug-in cars or used ones that have not previously had stickers. 

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The state government has made it clear that its purpose in issuing the stickers is to get more electric cars onto the roads. With congestion starting to choke even the diamond lanes, free access would be no incentive if everybody drove clean cars.

So in its latest bill to extend the HOV access program, the state legislature agreed to continue access for the most recent plug-in drivers, but to make the stickers expire.

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The new plan, along with the purple stickers, will provide a powerful incentive for electric-car fans to buy new cars and pass their old ones on to new owners when their stickers expire.

If you're buying a plug-in car in California before the end of the year, perhaps to take advantage of the last of Tesla's full federal tax credits, you may want to wait to apply for a carpool sticker until January 1, because you can get another full year of eligibility for the carpool lanes throughout 2022.

Update: An earlier version of this story misstated expiration dates for existing white and green California clean air stickers. They expire at the end of this year. We apologize for the error.