In the 18 years Barbara Kline has lived next to the interstate highway here, scores of marooned motorists have knocked on her door seeking help. But the knock early today signaled not a flat tire but the terrifying presence -- almost in her back yard -- of 11,000 gallons of liquid hydrogen.

Maryland State Police woke her family at 2:30 a.m. to warn that a tanker truck laden with the highly explosive chemical element had rolled down a 70-foot embankment from I-70 just behind their house.

"I was in such a state of shock, I thought it was a dream," she said. "Then I looked outside and saw all the equipment. The truck was 75 feet from our house."

Kline, her husband and her son were among 200 residents of this small community six miles west of Frederick who were evacuated by police because of the danger of an explosion or fire.

A 12-mile stretch of the highway between Hagerstown and Frederick was closed at 3 a.m. and stayed off-limits to traffic until firefighters and officials from Air Products Inc., the Pennsylvania company that owns the truck, could transfer the volatile, supercold liquid to another tanker.

By 3:30 p.m. traffic resumed on the highway, and fire and safety officials, many of whom had been on the site for 13 hours, were able to breathe a sigh of relief.

"Potentially, it was one of the most hazardous situtations we've faced," said Willam McLaughlin, who headed a Hazardous Material Response Team from Montgomery County that lent assistance to the Myersville Fire Department.

The truck driver, Thomas L. Johnson, 44, and a passenger, Harold L. Goff, 40, both of Newark, Del., suffered minor injuries in the accident, the cause of which is under investigation by police.

The truck, on its way to Delaware City, Del., ran onto the shoulder of the highway, slid down an embankment, overturned once and landed upright, police said.

Myersville Fire Chief Mark Sexton, one of the first to arrive on the scene, said he ordered the highway closed, fearing that the truck was leaking hydrogen. The gas is cooled to -454 Fahrenheit and is transported under great pressure in liquid form. McLaughlin said the 11,000 gallons in the truck could have produced a fireball endangering everyone within 1,500 feet.

As a precaution, police evacuated every resident within a half mile of the accident site.

Fire officials determined later that there was no danger that the gas would leak.

A spokesman at the company's Allentown, Pa., headquarters said that none of its trucks, which transport the gas in specially designed tanks, has ever leaked following an accident. Last year the company shipped 580 million gallons of liquid hydrogen, which has various industrial applications.

He said company officials advised against closing the highway.

About 60 people sought shelter at Red Cross emergency centers set up at two local fire departments. Others stayed with family and friends, said Paul Mossburg, chairman of the Frederick County Red Cross.

Most had other experiences to share about highway wrecks, but none topped today's.

"Off and on, this has been the story of our life . . . but this one was different for sure," said Kline. "Every time I hear a noise now, I'll be more frightened and alarmed than I was before."