California leads the nation in policies that promote reductions of carbon emissions from road vehicles.
It is unique in having a federal waiver that allows it to set its own, stricter emissions standards, along with a zero-emission vehicle mandate that requires automakers reaching certain sales volumes to sell a certain number of cars without tailpipes.
And where California leads, other states have followed.
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Thirteen states—mostly in the Northeast and Northwest—and the District of Columbia have adopted California's stricter emissions standards, many of which also participate in the zero-emission vehicle mandate.
Known as "Section 177" states, those 13 are: Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington.
Arizona adopted California's standards in 2008, but the state repealed them as soon as Governor Jan Brewer took office in January 2012.
Nine of those states also participate in the mandate to increase sales of zero-emission vehicles: Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island, and Vermont.
Together, the nine "ZEV states" and California represented 28 percent of new-car registrations in the U.S. in 2015, according to Automotive News (subscription required).
But while the nine states are nominally included in the ZEV mandate, automakers do not generally treat them differently than ones that are not.
That's because of a "travel provision" in the ZEV rules through 2018 that essentially allows carmakers to earn ZEV credits in every state for a zero-emission vehicle that's sold in any ZEV-Program state.
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That means sales are still largely concentrated in California, which has the most generous incentives and the most expansive charging infrastructure of any state.
California buyers can choose from a much larger selection of plug-in cars than those in other states—even those states with their own ZEV mandates.
Sales of hydrogen fuel-cell vehicles are also limited to California at this time, as it is the only state with any substantial fueling infrastructure.
2013 Honda Fit EV drive event, Pasadena, CA, June 2012
But the ZEV mandate has had some impact on electric-car sales outside California.
The discontinued Honda Fit EV and Chevrolet Spark EV—both "compliance cars" built solely to satisfy the mandate—were eventually made available in at least one or two other states.
Markets for certain limited-volume electric cars—such as the Mercedes-Benz B250e and the Volkswagen e-Golf—also roughly correspond to the ZEV contingent.
CHECK OUT: California bought more electric cars than rest of U.S. combined in June (Jul 2016)
Because the California standards are the only alternative to federal standards that states may adopt, carmakers have a relatively large, stable bloc to plan emissions compliance around.
The continued existence of separate emissions standards for California and its followers relies on waivers granted by the EPA.
To date, requests for approval or renewal of those waivers have never been denied by the agency.
U.S. Capitol Building
However, the current administration is expected to take environmental policy in a radically different direction than its predecessor.
The current EPA administrator, Scott Pruitt, denies accepted climate science and sued the EPA more than a dozen times in his previous position as Oklahoma attorney general.
Thus far, the EPA has not taken any public position on the continuation or revocation of California's emissions waiver.
Unattributed recent media reports, however, have indicated that the EPA plans to attack the waiver, to eliminate California's ability to set its own emission standards.