Trucker Dave Lengacher had his 18-wheeler in the passing lane of Rte.I-81 in western Virginia last May when he was startled to see headlights dead ahead through his windshield.

"They were in my Lane and heading right for me," said Lengacher, 24, with disbelief, as he recalled what happened next.

Lengacher's multi-ton tank truck crashed head-on into a car driven by Walter Steele of Clear Springs, Md., shearing the passenger side from the car and killing Steele instantly.Lengacher's load of nitro-benzol, a chemical used to make dyes, "spilled al over the road and could have exploded," said a state trooper who was at the scene.

According to police, Steele was driving over the 55 mph speed limit and was headed south - the wrong way - down the highway's northbound lanes.

States and federal highway officials and police spokesmen say such collisions - many involving drunk drivers - are not unusual and demonstrate in grisly fashion that a foolproof solution is yet to be found for preventing such mishaps.

"It's a serious problem," said Virginia State Police 1st Sgt. Herbert D. Northern. "All last year we had four incidents involving wrong-way drivers and no fatalities in Fairfax County. We've already had five this year and one fatality."

A Maryland State Police spokesman said Montgomery County recorded 1k such accidents and one death in 1978, "and the figure is higher this year. It looks like we're way ahead of ourselves."

Officials, who say intoxication and confusion are two major cause of wrong-way driving, say that clearly marked signs, arrows on the pavement and increased police enforcement all have failed to wipe out such accidents.

Among the cases they cite:

Last January a Fairfax City resident was involved in a hit-and-run accident on Rte.I-95 at about 4:15 a.m. The man, who was driving south in the northbound lane, was "too intoxicated to tell us how long he had been driving that way," said a state trooper who caught up with him.

On Feb.6, a Bowie man driving on I-395 near the Springfield exit decided, in the midst of morning rush-hour traffic, that he was going the wrong way. He was stopped shortly afterward by a state trooper, alerted by another driver with a citizen's band radio. The man told the trooper be had been "scared" while driving for a mile against traffic.

On April 3, an intoxicated Alexandria woman was driving west on the Capital Beltway when she missed an exit. Making a U-turn she traveled three miles against heavy traffic before she was apprehended by state police.

Other incidents are more serious. Blair Kirkland Horton, 32, of Springfield, was hospitalized June 24 after colliding with a motorcyclist headed south without headlights in the northbound lanes of the beltway near Tysons Corner. The cyclist, Stanley Clayton Darnell, 21, of Princeton, W.Va., was killed instantly.

Officials say that most frightening aspect of accidents involving drunk drivers is that the offenders seem to be completely unaware of any wrong-doing.

A woman who drove south for eight miles in the northbound lanes of I-395 last November said she "couldn't understand why all those other cars were going in the wrong direction," according to police.

"You can call it the alcoholic rationale," said Barbara Grimm of Virginia's Alcohol Safety Action Program. "A 'wrong way' sign just won't register. They'll believe that they're in the right. It's a complete loss of rational thought."

Officials of the National Transportation Safety Board, the National Highway Traffic Safety Administration and the Federal Highway Administration said the latest study of wrong-way driving was done in California in 1968.

An analysis of that state's highway system led officials to conclude that little could be done to prevent intoxicated drivers from traveling in the wrong lanes.

"Even active signs, those that trigered bells and whistles, would not stop them," said Charles Scheffey, director of research for the FHA. "If the person is drunk, there is hardly anything that will keep him off the highways."