This time, the deadly traffic duel began on a busy commuter route just south of Andrews Air Force Base, with one angry man in a blue Nissan Sentra, the other in a black, sporty Mercedes-Benz Kompressor.

What ignited the dispute that boiled into road rage that recent Wednesday isn't clear. But the incident lasted only a few minutes in the twilight in Prince George's County. And once again, blood was spilled on a local roadway not by accident or through simple carelessness but as a result of aggressive driving.

For years in this growing region, where more and more frustrated motorists are spending more and more of their hectic lives stalled on congested thoroughfares, officials have sought to stem the violence that erupts occasionally. Laws have been enacted, public awareness campaigns have been waged and concerted enforcement efforts have been carried out by police in Maryland, Virginia and the District.

From the data compiled by area law enforcement officials, it is difficult to determine whether aggressive driving has declined. But highway safety activists have said that based on what they see daily on the roads, rage continues to be a serious problem.

"If anything, it appears to be growing worse," said Peter Kissinger, president of AAA's Foundation for Traffic Safety. "There are more cars, more traffic. There's more multitasking. There's more concern over time."

In the Oct. 12 Prince George's incident, "Essentially they were weaving in and out of traffic, and one cut off the other," a law enforcement official said. "One guy starts shouting profanities at the other guy. They're both [angry]. And it ratcheted up all of a sudden."

It came to a head at a traffic light at Old Alexandria Ferry Road and Virginia Avenue in Clinton.

As other motorists watched, the two drivers stopped -- Alfred L. Evans, 39, in the Mercedes, and Keith R. Ingaharra, 28, in the Nissan.

It might have ended differently, with just angry words, or maybe a punch or two thrown. It might have been just another scuffle between motorists -- like countless others in traffic-choked metropolitan areas nationwide -- if neither driver had been armed.

But one had a .45-caliber Glock.

A Tragic Turning Point

Aggressive driving and the road rage that sometimes results have long ranked high on the list of traffic-safety issues that worry police and highway officials. If there was a moment when the phenomenon also became a top concern among Washington area motorists, Kissinger said, it was on a spring morning nine years ago.

On April 17, 1996, two men, both 26, engaged in an angry, seven-mile duel on George Washington Memorial Parkway in Virginia, swerving from lane to lane and cutting off other vehicles at speeds reaching 80 mph. Their road rage ended in a horrific crash near McLean that killed one of the men and two other motorists, neither of whom was involved in the dispute. The surviving aggressive driver got a 10-year prison term.

That highly publicized tragedy prompted Kissinger's foundation to commission a study of aggressive driving. The expert hired for the project, Louis R. Mizell Jr., a Bethesda-based crime-data researcher, found 10,037 incidents nationwide from 1990 to 1996 in which aggressive drivers intentionally killed or hurt people or had tried to do so.

More than 200 people died in the incidents, and more than 12,000 were injured. They were shot, stabbed, beaten and run over, among other modes of attack.

"One thing you'll find is, it's usually an accumulation of stressors" that pushes some motorists to road rage, Mizell said in an interview. "It's like the old saying about the man who finally snaps. It's not the divorce he just went through or the job he just lost" that sends him over the edge. "It's the shoelace that breaks. That's the final thing."

On the road, the final straw could be another motorist who cuts a driver off, or an inattentive driver sitting through a green light. And although the most common aggressive drivers are drug-using young males in their late teens and early twenties who have criminal records, Mizell said, the 10,037 incidents he analyzed involved all sorts of volatile motorists.

He wrote in his report that "hundreds of aggressive drivers -- motorists who have snapped and committed incredible violence -- are successful men and women with no known histories of crime, violence or alcohol or drug abuse. When the media interview the friends and neighbors of these individuals, they hear that 'he is the nicest man,' 'a wonderful father,' or 'he must have been provoked.' "

"You show me the kind of people you think are not going to do this," Mizell said recently, "and I'll give you examples where they did."

After the 1996 parkway crash, Maryland and Virginia legislators recognized that aggressive driving is a precursor to most road-rage incidents, Kissinger and other highway safety advocates said. The two states passed laws making aggressive driving a specific traffic offense. Eight other states have enacted similar statutes; the District has not.

Also as a result of the crash, 18 police departments in Maryland, Virginia and the District joined in a program called Smooth Operator, an enforcement and public awareness campaign against aggressive driving that began in May 1997 and now includes more than 80 police agencies in the region, said Thomas Gianni, the Maryland highway safety official who coordinates the annual effort.

For one week each May, June and July and for two weeks each August, police departments in the program devote all available resources to a crackdown on motorists who speed, disregard traffic lights and signs, tailgate or pass other vehicles illegally. The goal, Gianni said, is to prevent road rage by targeting individual traffic violations that, when committed together, usually constitute aggressive driving.

The participating departments issued more than 260,000 citations in summer 2004 and more than 390,000 this summer, Gianni said, although "a very small fraction were for aggressive driving specifically. Speeding is the main violation." Each enforcement wave is accompanied by a blitz of TV and print advertisements.

Whether Smooth Operator has helped curb aggressive driving is difficult to measure accurately. "In our telephone surveys, when we talk to people who'll own up to aggressive driving, we're surprised to find that a lot people, especially younger drivers, find it a competitive thing," Gianni said. "Sometimes it's not necessarily frustration, but the thrill of getting ahead of the other guy."

In that sense, he said, it is clear that the program has not achieved one of its important goals: changing the mind-set on the region's roads to make aggressive driving as socially repugnant as drunken driving.

"We haven't reached that point yet, to where it's stigmatized," Gianni said. "And anybody who drives on the highways can see we have a long way to go."

Dueling, and Then Death

Ingaharra and Evans, both of whom lived in Prince George's, were not the most law-abiding of drivers, according to court records. Each had been cited for at least a half-dozen traffic offenses, records show -- never for aggressive driving specifically, but a few times each for speeding.

Evans, a field technician for Washington Gas, had been arrested twice in 1990 for allegedly carrying a gun illegally, authorities said. They said Ingaharra, who was unemployed, had been the subject of several criminal complaints that accused him of harassment and making threats.

The two were strangers when they dueled on Old Alexandria Ferry Road.

"They were arguing back and forth in their cars," said a law enforcement official, speaking on the condition of anonymity because the incident is being investigated. "When they got to the intersection, [Ingaharra] got out, started walking toward the other guy with his arms up, like, 'What do you want to do now?'

"Basically he was egging him on," the official said.

Evans's attorney, David Simpson, said in a brief interview that his client was afraid of Ingaharra. Simpson acknowledged what Evans did next: He pointed his Glock out the driver's window of the Mercedes and squeezed the trigger, again and again.

Ingaharra, of Clinton, who police said was not armed, collapsed to the pavement, wounded several times in the lower body. He died in a hospital eight days ago and was buried Saturday.

Evans, of Upper Marlboro, charged with first-degree murder, intends to plead not guilty and argue that his actions were justified, Simpson said.

At a court hearing in Prince George's two days after the shooting, a woman who identified herself as Evans's mother told a judge that the incident was out of character for her son. "He's an excellent husband," she said.

"He works hard. . . . He's not a risk or threat to society."

Staff writer Michael E. Ruane and staff researcher Bobbye Pratt contributed to this report.