A 50-year-old truck driver took what police described as a "heroic" leap in his 18-wheel tractor-trailer yesterday morning, crashing through 170 feet of guardrail on the Woodrow Wilson Bridge, plunging 35 feet into Virginia and walking away virtually unharmed.

The driver, Robert Smith, averted what Virginia State Police said could have been a major rush-hour accident and survived with only minor injuries.

"It was amazing," said Officer Dean Jones, "I've never seen anyone come out of anything like that alive . . .'

Smith's truck landed nose first next to South Royal Street in Alexandria, shattering the Fiberglas body of the cab and breaking the truck into four pieces.

"The only thing left intact was the driver's seat," said Jones.

Smith, a resident of 547 Randolph St. NW who has been driving trucks for 30 years, said he was driving southbound in heavy traffic when a car suddenly pulled in front of him and slowed.

Smith said that to avoid hitting the car, he swerved into the bridge.

The truck climbed the guardrail, then toppled to the ground below, pulling the airborne trailer on top of it.

"The only reason Smith lived," said Jones, "was because the truck was only partially loaded. If he had been carrying a full load, he would have been dead."

Smith crawled out of the wreck himself. He was taken to Alexandria Hospital where doctors closed a cut on his right leg with eight stitches. He was also treated for contusions to his left shoulder and leg.

Less than two hours later, he was back beside his truck, a crumpled mass of steel lying belly-up. "I wanted to make sure nobody else was hurt," he said.

Police said Smith wept when he saw truck again. "He couldn't believe he was still alive."

At four o'clock yesterday afternoon, as employes of the truck's owner -- Preston Trucking Co. in Ardmore, Md. -- were towing the last pieces away, Smith sat in a company car nearby and stared into space.

"The only thing I could see was tombstones and my family," said Smith of his fall from the bridge. "My daughter [23] just got out of the hospital with a nervous condition. I could just see her goin' right back in if anything happened to me."

Smith sat with his hands folded in his lap. He wore a blue cap. His blue work shirt was torn at the sleeve.

"I was just tryin' to say a fast prayer," he said.

Smith said the driver who cut in front of him did not stop. Virginia State Police said an investigation of the accident is underway.