Two tractor-trailer accidents snarled traffic on the Capital Beltway yesterday, one late in the morning rush hour in Virginia, the other early in the afternoon in Maryland.

A 40-year-old truck driver escaped with only minor injuries in the morning accident when his tractor-trailer hit a median strip, jackknifed, and plunged down a 15-foot high wall onto the outer loop near Annandale.

About six hours later, Maryland police said that a tractor-trailer flipped over as it came onto the beltway from Rte. 50 near Landover. The 2:30 p.m. accident blocked two lanes of the inner loop for two hours, police said.

Police said no other vehicles were involved and the truck driver, who was not immediately identified, received only minor injuries.

The spectacular 9 a.m. accident in Virginia tied up traffic for about a mile in either direction from the accident, between Braddock Road and Little River Turnpike. Virginia state police said the congestion would have been worse except for their recently increased patrols on the area's interstates, which allowed nine troopers, instead of the usual one or two, to arrive at the scene within minutes.

Police said the accident occurred as Gerald Broughman of Staunton drove an empty Smith's Transfer Corp. tractor-trailer at about 35 miles per hour in the left lane of the inner loop.

State Trooper K.W. Evans said a witness told him that a car tried to cut in front of the truck and that the truck ran onto the emergency shoulder to avoid the car, but struck the concrete median barrier. The truck then jackknifed, flipped over the median, and dropped about 15-feet into the left lane of the outer loop, he said.

The Fairfax County Board of Supervisors, calling beltway tractor-trailer accidents "one of the most serious problems facing Northern Virginia," recently asked Gov. Charles S. Robb for a comprehensive strategy to reduce accidents and relieve resulting traffic jams.

The Virginia Department of Highways and Transportation Commission recently voted to ban trucks from the left lanes of a 6.5-mile portion of the Beltway between the Shirley Highway and U.S. Rte. 1 near the Woodrow Wilson Bridge 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 Virginia accident was not in the section of the beltway covered by the truck ban.

A recent American Automobile Association study found that while trucks accounted for only 4 percent of beltway traffic in Virginia, they were involved in 16 percent of the accidents. Trucks were involved in 166 of the 448 accidents on the beltway during the first nine months of this year.