Maryland will continue indefinitely to prohibit trucks in the far left lane of 31 miles of the Capital Beltway, and Virginia will extend its ban on a six-mile stretch for another year, state officials announced yesterday.

In addition, Maryland officials probably will expand the ban to include all 41 miles of its portion of the Beltway when widening to eight lanes is complete in a few years. They also are studying a truck restriction in far left lanes for all the state's eight-lane freeways, according to Thomas Hicks, deputy chief engineer of the State Highway Administration.

The two states began restricting truck traffic in the left lane along about half the Beltway's length in December 1984 in a one-year experiment to see whether traffic safety was improved. Supporters cited figures showing tractor-trailers accounted for about 16 percent of the accidents, but only about 4 percent of the traffic on the road.

Separate studies in each state over the last year show a slight improvement in safety in Virginia and no change in Maryland, but officials said an important reason for keeping the restrictions is that the public thinks the Beltway now is safer.

"The intimidation factor seems to have lessened," said Lynda J. South, a spokeswoman for the Virginia Department of Highways and Transportation. The ban was extended for a year to allow further study of the safety impact, she said.

"We found there's not a whole lot of safety basis for keeping it in, but there certainly is a public desire that's been exhibited to us for maintaining it, so we have no problem with that," Hicks said.

American Trucking Association official William E. Johns said his group, which initially opposed the ban, wants to see the safety data before commenting on the decision to continue it. "If public opinion is wrong, what's the basis for their decision?" he asked. "If the safety data show an improvement, we support it."

Mary Anne Reynolds of the American Automobile Association said her group supports "anything that makes motorists feel better about driving on the Beltway. Anything that can be done to ease the frustration level may cut down on people taking dangerous chances."

Fairfax County Board of Supervisors Chairman John F. Herrity, a supporter of the Beltway truck regulation, said he is pleased at Virginia's decision to bar trucks from the high-speed lane. "They scare the hell out of me," he said.

The Virginia left lane restriction includes part of the Beltway between Rte. I-395 to the Woodrow Wilson Bridge. Maryland bans truck traffic on the left lane between the bridge and Georgia Avenue. On the Virginia part of the Beltway where trucks are banned from the left lane, the number of accidents stayed about the same, but the accident rate declined slightly and the severity of injuries was down 20 percent, South said.

There were 101 Beltway accidents in the year before the restriction, and 102 the year after, during a period when traffic volume rose 8 percent, South said. Some of the accidents did not involve trucks, but she did not know how many.

The accident rate dropped from 49 per 100 million vehicle miles of travel to 46, South said.

In Maryland, Hicks said overall accident rates are about the same since restrictions on truck traffic began, but rear-end accidents involving trucks were reduced, and side-swipe accidents involving trucks increased. "It's a trade-off," he said.

Hicks said the severity of accidents stayed about the same, and so did the number of accidents, although traffic volume rose by 3 to 5 percent. Staff writer Barbara Blechman contributed to this report.