The two drivers had been battling for about four miles, jousting for position in the heavy rush-hour traffic streaming homeward from Birmingham along southbound Interstate 65. After one vehicle cut off the other one, they played a cat-and-mouse game, tailgating, lane-changing, slamming on brakes until they got off at the same exit.

Gena Foster, 34, was racing to pick up her daughter Francie, a 4-year-old with a mop of blond hair, at an after-school program for children with cerebral palsy. Shirley Henson, 40, was on her way home to her husband and dogs in a quiet cul de sac. But when the two cars came to a stop at a traffic light on the darkened exit ramp, Foster jumped out and started toward the immaculate black sport-utility vehicle idling behind her.

Inside the Toyota 4-Runner, Henson reached into the console next to the seat, where she kept a .38-caliber revolver and a cell phone. As Foster approached her door, Henson lowered the window about halfway and reached for the revolver. She fired a single shot, striking Foster in the left cheek. Foster crumpled to the pavement, blood gushing from her face, dying. She never made it to school.

Nor did Henson, a secretary at a prominent construction company, ever reach her home in one of the countless new subdivisions that have transformed Shelby County over the last decade from a hilly expanse of dairy farms and limestone quarries into one of the fastest-growing counties in the South. Instead, she spent the night in the county jail charged with murder. Out on $50,000 bond, she now awaits a Dec. 1 court hearing.

But while the Nov. 8 killing was unprecedented in the upper-middle-class community of Alabaster--consistently ranked as the safest place in Alabama--authorities say it shouldn't come as such a surprise. The county's explosive growth has turned a 20-mile commute to downtown Birmingham into a roughly hour-long ordeal of stop-and-go traffic, and guns are easily accessible. Law enforcement officials estimated that at least half of all motorists in this part of Alabama--and perhaps significantly more--keep firearms in their cars.

"I expect there'll be more situations such as this because of heavy traffic and the guns being so prevalent and people not knowing when to use them and how to use them," said Police Chief Larry Rollan. "Everybody's got a gun."

"Both people were probably stressed out," said John Ward, state president of the National Safety Council. "Birmingham is growing, especially Shelby County, and the roads haven't been able to keep up. There's a lot of tension and pressure when you have bumper-to-bumper traffic."

But there's another reason it has become the buzz, consuming dozens of hours on the local talk radio and sparking debate at the Waffle House that's as hot as the coffee, even rivaling the breathless banter about the Shelby County Wildcats' march toward the state high school football championship.

"They're both middle-class women, both responsible," Shelby County District Attorney Robby Owens said, noting that neither had police records. "You're not dealing with two guys with short fuses and a lot of testosterone going on. You're not dealing with two kids who tend to overreact."

The shooter, Henson, is a former Cub Scouts den leader with an 18-year-old son away at college. The victim, Foster, was the mother of three children who earlier this year moved into a new home on three acres of land, where she was keeping two horses and teaching her teenage daughter to ride.

As in booming suburbs elsewhere, the roads here have failed to keep pace with the bumper crop of new houses rising from the bulldozed fields of Shelby County, which is to Birmingham much like Loudoun County is to Washington. Shelby has added 100,000 more residents in little more than 10 years, many of them leaving Birmingham. In Alabaster, where the population has doubled in the last decade, Police Chief Rollan recalled that until about 15 years ago the only traffic to speak of occurred on the day of Alabama-Auburn college football games in Birmingham. On those rare Saturdays, police operated the only traffic light by hand.

Now, dozens of traffic signals in Alabaster wage a losing battle with congestion and I-65 is bumper-to-bumper every weekday morning and evening. Traffic on the interstate near Alabaster has doubled in the last 15 years but the highway remains only two lanes in each direction.

As congestion has followed in the wake of sprawl around the country, so has road rage, according to the AAA Foundation for Traffic Safety. It reported that these incidents have increased nearly 60 percent from 1991 to 1996, resulting in 218 deaths over that period. In only 4 percent of the cases, however, were women involved.

A study this year by the Surface Transportation Policy Project, a research and advocacy group based in Washington, reported that aggressive driving was more frequently associated with the stress of driving in sprawling suburbs than in older urban areas. The analysis found that Alabama ranked third in the country in the rate of death from aggressive driving. (This figure includes fatal crashes involving aggressive behavior though not roadside murders.)

Henson kept the revolver, for which she had a permit, stashed in her SUV for protection at the urging of her husband and brother, according to her lawyer, David Cromwell Johnson. Few here seem to fault her for keeping the .38 close at hand. "You take away someone's handgun only by prying it out of their icy dead grip," said Kevin Miller, host of an evening talk show on WERC radio in Birmingham. In fact, nearly half his callers support Henson for pulling the trigger.

"If I'm in my car and somebody comes running up to my car, I sure would shoot them. I'm sorry, but that's just the way it's gonna be," said a caller named Becky. Another caller, Bobby, who also carries a gun in his car, said: "You don't know the intent of the lady that was coming at her. . . . If I believe they're going to harm me, they're going to get shot."

In the mornings, Henson often left her two-story house, with its white-railed front porch and high red shutters, before her husband, Mike, set out in his Jeep Cherokee on a shorter commute to his job as a salesman for a building materials company. She would race home in the afternoon after leaving her job at Harbert Corp. to train her two beloved labs, JD and Emma. She and Mike have been grooming them for competition, teaching them to run slalom courses through 10 white poles arranged beside the long driveway.

Raised in the countryside south of Birmingham, Henson moved with Mike and their son, Steven, to Alabaster about a decade ago. It was a time when many office employees of BellSouth Corp., the University of Alabama at Birmingham and regional medical centers were crowding into Alabaster, once known primarily for the limestone quarries and lime plants that gave the town its name.

Foster, by contrast, was a transplant from San Diego. Barely 20 at the time, she fell in love with her future husband, Chris, during a month-long vacation in Alabama and quit her lucrative job waitressing at an elite California golf course to move east. Last winter, she realized the dream of a dozen years, a neat three-bedroom prefabricated house on three acres of land off a narrow country byway called Looney Road in Columbiana in Shelby County. There, she would keep her horses, Daisy and Lady.

"She finally got what she wanted. She finally got it all in order," said brother-in-law Shane Foster, 22.

In the living room, she mounted the heads of three deer, two killed by Chris, and installed a grandfather clock, a computer for the children and a piano for 13-year-old Brittany. On the refrigerator, beside a fish tank, she had hung pictures of the children, emergency telephone numbers and a pair of magnets boasting: World's Greatest Mom and Mother of the Year.

While Gena and Chris Foster divorced two years ago, they remained close. They went together to the Shelby County High School football games to watch Brittany play flute in the marching band and to the junior football games in which their 10-year-old son Christopher plays outside linebacker. On the fateful Monday, with her own Chevy Blazer in the shop for transmission repairs, it was her ex-husband's black Pontiac Grand Prix Gena was driving.

That evening, Foster left her job at CMS Field Products, located on a wooded campus of modern two-story buildings, at about 5 p.m. Ordinarily, she worked assembling sensors that monitor air quality. But she had spent that day packing up the company's offices for an upcoming move to another nearby location and, co-workers said, was in a particularly jolly mood.

As Foster headed out into the dusk toward I-65, Henson was leaving her job at Harbert Corp. atop a forested hilltop less than two miles away. They reached the interstate at nearly the same time and pulled out into heavy but moving traffic.

About two exits to the south, Foster jutted into the left lane, cutting in front of Henson and almost clipping the SUV, said Jim Hardy, who was driving behind them. Henson seemed to flash her headlights at Foster, he said. Foster hit her brakes. "They go back and forth. The 4-Runner pulls up on the bumper and then gets back," Hardy said. The contest continued for four miles, police said, as the traffic wound past silhouetted pine trees and shadowy hills.

Both vehicles left the interstate at the Alabaster exit, a straight ramp that slopes upward to a traffic light at an overpass. Henson moved into the right lane, preparing to turn toward Alabaster. Foster, who most evenings would move into the left lane and turn toward Columbiana, also pulled to the right, stopping her Pontiac in front of Henson, police and witnesses said.

Foster bolted from her car, leaving the door open. She headed back to the SUV, parked about seven feet behind. Her arms were out. She was yelling something no one could hear. "She was mad. Her eyes were wide open," Hardy said. As other cars pulled up to the traffic light, Foster came up to the partly open driver's window.

Since the incident, Foster's family has figured she approached Henson to put an end to the confrontation. "Gena was the kind of person who wanted to straighten everything out. 'What's going on here? Let's stop it here,' " surmised her sister, Kimberly Pedigo.

Henson's lawyer, Johnson, said she saw it differently: "Shirley thought the lady had something in her hand, a knife or gun, and thought the lady was going to kill her. The lady gets close enough where she can spit in Shirley's face. Shirley almost reflexively, afraid to death, scared to death, she shoots the lady."

Foster dropped. Blood gushed from her face, painting a swath down the ramp nearly two feet wide. Henson dropped the revolver onto the passenger seat beside her briefcase. This time she reached for the cell phone and dialed 911. But she quickly became hysterical.

She remained frozen in her seat, weeping, said Lisa Adney, another motorist who helped her complete the call. Henson glanced at the body by the door and quickly looked away. "Oh my God, I shot her," she repeated over and over. "Oh my God, I can't believe I shot her. Oh my God, I can't believe she's dying."

CAPTION: Gena Foster, shown with her ex-husband, Chris, and children Christopher, left, and Brittany, was shot by another commuter on her way from work.

CAPTION: Shirley Henson, of Alabaster, Ala., is charged with the roadside killing of Gena Foster.