A tractor-trailer nose-dived 24 feet from a Shirlington overpass onto the northbound lanes of Shirley Highway during yesterday morning's rush hour and miraculously, according to Virginia State Police, hit no other vehicles.

The accident occurred about 9 a.m. when the truck went out of control on the exit ramp to Shirlington from the northbound lanes, ripped through 40 feet of the left guardrail and plunged off the northern bridge of the traffic circle into the traffic lanes below, police said.

The truck driver, John Lewellyn Smith, 40, of Manassas, suffered multiple injuries but was listed in stable condition in Alexandria Hospital. The accident was the latest in a recent series of major truck accidents that have plagued heavily traveled commuter routes in the Washington area.

State police said they could not offer an explanation for yesterday's accident, which backed up traffic on the four northbound lanes of Shirley Highway (I-395) from the overpass at Shirlington to Duke Street three miles away.

"It's an absolute miracle," said state police Sgt. Dennis Robertson. "Most days at 9:08, the traffic is dead still. It's stop-and-go, heading into D.C. I think people slowed down because they saw one of our troopers writing a ticket near the circle," Robertson said. "They were alert and I think it might have kept them from being hit."

When the 45-foot empty trailer broke through the guardrail it tore open its diesel tanks. A few small fires were reported. Highway department officials immediately closed Shirlington Circle until the Alexandria Fire Department mopped up the 15 to 20 gallons of spilled fuel.

"All I heard was this crashing sound," said state trooper Joseph Harvey, the officer who was writing a speeding ticket near the scene of the accident. "It went bounce, bounce, bounce against the rail and then -- bang -- right onto the highway."

Cars heading for Washington sped up, slowed down, or swerved to the right. "I could hear the screeching brakes," the trooper said. There was nothing left of the cab; all I could see was the wheelbase," he said.

The tractor-trailer, owned by Skippy Trucks Inc. of Fairfax and carrying only a few paper bags, flopped onto a median and stuck out only a few feet into the left express lane.

For all but two 15-minute intervals while the cab was removed and the overpass inspected, Shirley Highway's right lanes remained open. The highway's normally restricted express lanes were opened until noon to vehicles with fewer than three riders in an effort to ease the congestion.

The incident yesterday was part of an alarming increase of tractor-trailer accidents on the Capital Beltway. There were 198 in the first six months of this year alone compared to 208 for all of 1983, according to the American Automobile Association. On Monday those figures prompted the Fairfax Board of Supervisors to renew its 1983 call for a ban on large trucks from the left lanes of the beltway.

Trooper Harvey said he believed the fact that he was writing a ticket probably had slowed the traffic. "People were paying attention to what was going on because they saw me . . . " he said. "They were definitely moving slower than usual. And for some reason the rush hour had already rushed itself out."

When the trooper reached the truck, two people had pulled Smith from the cab. A hospital spokesman said Smith suffered a broken wrist, jaw, and leg.

The ticket Harvey was writing was for a woman whom he had clocked at 67 mph in a 55 mph zone. "She said she was late for work," the state trooper said, "and I had just told her: 'It'll do you no good if you never get there.' "

Harvey shook his head. Only a few months on the job and celebrating his 32nd birthday, he said: "Life is mighty strange." He never finished writing the ticket.