The Virginia Highway and Transportation Commission reversed itself today and voted to ban trucks from the left lanes on a 6.5-mile portion of the Capital Beltway for a one-year period, beginning Dec. 1.

The ban will coincide with a similar one-year experiment on a 31-mile stretch of Maryland's Beltway, between the Woodrow Wilson Bridge and Georgia Avenue (Rte. 97). The tests are an attempt to see whether restricting truck traffic to three lanes in the heavily-traveled section between Shirley Highway and U.S. Rte. 1 near the Woodrow Wilson bridge will reduce the number of truck accidents on the Beltway.

The commission vote will not affect truck traffic on the Beltway between the Cabin John Bridge and Shirley Highway.

Despite heavy lobbying by Northern Virginia officials, a divided highway commission last month rejected the test ban in a 5-to-4 vote, with State Highway Commissioner Harold C. King casting the deciding vote. The action provoked an outcry from suburban Washington officials, who said the vote reflected the insensitivity of Richmond officials to one of Northern Virginia's worst traffic problems.

King today apologized for the first vote, telling the commission "I was wrong . . . . I voted against it. And then the other shoe dropped. It is an extremely emotional issue . . . but I felt I was representing commission members who were not present."

A Northern Virginia member of the commission, Joseph M. Guiffre of Alexandria, absent last month because of a death in his family, told the commission this afternoon: "I strongly support the test."

Guiffre made the motion to restrict trucks for a year, which carried 7-to-2, with King supporting the ban. H.R. Humphries of Weems and Robert A. Quicke of Blackstone opposed the ban as an unfair restriction on truck traffic and noted that Virginia State Police have said it will be difficult to enforce.

State Police this week announced they are tripling the number of troopers patroling the Beltway in a month-long experiment to see if they can help cut traffic accidents.

A recent study by the American Automobile Association found that while trucks last year accounted for only 4 percent of Beltway traffic in Virginia, they were involved in 16 percent of the accidents. More than 100,000 vehicles a day travel the Beltway.

Some highway commission members said the proposed truck ban should have relatively little impact on truck traffic since statistics show that only to 4 to 8 percent of the trucks using the affected section drive in the left lanes, said commission member Eugene Smith of McLean. Smith fought vainly for the Beltway truck restriction last month.

The ban affects all trucks with six tires, but excludes recreational vehicles. It does not actually extend to Woodrow Wilson Bridge but only to the Virginia interchange, about 100 yards from the bridge, where the highway narrows from four lanes to three.

The commission will review Beltway accident statistics every two months and will reconsider the ban if, as King said, truck accidents "increase dramatically." Officials will make a final report on the ban's effectiveness at the end of the 12-month trial period.

Commission member Edgar Bacon of Bristol, who switched his vote today after he'd gotten a lot of letters from angry motorists who want the truck ban, said he did not like the commission "making laws" restricting truck traffic.

King answered that the commission was merely establishing the rules of the road, which all states do. Maryland already bans trucks from inner lanes of portions of I-270 and the John F. Kennedy Memorial Highway, which carries I-95 traffic north of Baltimore. Several other states have imposed similar truck bans on interstate highways, such as on I-66 inside the beltway.

King said "we don't have any reason to know this ban is going to be helpful, but the resolution of the Fairfax County Board of Supervisors wants it and so many people believe this will help."

Smith, a strong proponent, said the trial ban "is an experiment . . . not a panacea. But we cannot not do anything that might improve safety."