Just when Rep. Stan Parris (R-Va.) thought he had a plan in his grasp to ease the much-debated car-pool restrictions on Shirley Highway, the District of Columbia and his own state government swooped in to snatch it away.

Parris said a plan to open the four-person car-pool lanes to all traffic during off-peak periods was set to go into effect in the coming months. It was based on legislation he won during the final minutes of the 1983 session.

But an amendment, inserted in the $7.2 billion highway bill approved by the Senate late Thursday, would change all that, he said.

"It was an end run by the District and the Virginia Department of Highways, which never wanted those express lanes open," an angry Parris said when he learned of the offending measure. "The District came in the steely night hours with this clandestine exercise to rewrite the provision . . . . It's asinine."

Parris' measure ordered the federal Department of Transportation to conduct a one-year project easing the controversial HOV-4 rule that restricts expressway lanes on I-395 at all times to vehicles carrying at least four passengers. Under his plan, the restrictions would apply during three-hour periods in the morning and evening, with the lanes open to all cars at other times.

The amendment, won by the District when the highway bill came through the Senate Environment and Public Works Committee, gives responsibility to the District and Virginia to conduct the project.

If the District were in control, its officials and those in Virginia would prefer that the lanes not be opened to unrestricted traffic, according to Sarah Campbell, federal affairs director of the D.C. Department of Public Works. Instead, Campbell said, only vehicles carrying two or more passengers could use the lanes even at off-peak times.

Campbell said the District and Virginia wanted control of the project because the local jurisdictions, and not the federal government, would be responsible for any problems or accidents that occurred because of the new plan.

How did D.C. win the amendment? "Let's just say that even though we're not represented in the Senate, D.C. is lucky to have some good friends there," Campbell replied.

Aides to members of the Senate committee said the committee leadership endorsed the new language, but did not intend that it should scuttle the previous plan. "We will monitor it to see what happens," said one aide.

The fight has just begun. The Senate and House have passed drastically different highway bills this session, and unless they can come to agreement in a conference committee, the 1984 highway legislation will die, and with it the D.C.-inspired amendment.

"If they go to conference on this," Parris promised yesterday, "I'll go to war on their amendment."