WASHINGTON — An ambitious plan by Eastern states for a regional cap-and-trade program to curb greenhouse gas emissions from cars and trucks got off to a slow start Monday after just three states — Connecticut, Massachusetts and Rhode Island — plus Washington, D.C., formally agreed to adopt it.

The program’s backers had originally aimed for broader participation and expressed hope that more states might join later. Last year, 11 Northeastern and Mid-Atlantic States, making up a fifth of the United States population, signed on to a draft version of the plan, which would set a cap, to be lowered over time, on the total amount of carbon dioxide that can be released from vehicles that use gasoline or diesel for fuel.

But so far, only a few states have said they would begin implementing the policy. In a separate statement on Monday, eight other states left open the possibility of joining at a future date, but would not commit for now. Those states include Delaware, Maryland, New Jersey, New York, North Carolina, Pennsylvania, Vermont and Virginia.

Under the cap-and-trade program for cars and trucks, which would start in 2023, fuel companies would buy allowances from participating states, either directly or on a secondary market, for every ton of carbon dioxide their fuel will produce. The states would then invest the proceeds into efforts to reduce emissions from transportation, such as trains, buses or electric-vehicle charging infrastructure.

Still, the ultimate effects of the vehicle cap-and-trade program may hinge on how many states end up joining, analysts said. The four jurisdictions that joined on Monday account for less than 3 percent of the nation’s transportation emissions, while the eight states that are considering their options account for another 18 percent.

“Right now many states are really focused on their Covid-19 responses and the economic recovery, which is demanding a lot of attention from governor’s offices,” said Jordan Stutt, carbon programs director at the Acadia Center, a research and public interest group in New England that is pushing for cleaner energy. “Now that the program’s moving forward, I do think we’ll see more states jump aboard, but I don’t want to make any assumptions just yet.”

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