On a recent Friday morning, David LeBlanc donned his Army uniform, kissed his wife and four children goodbye, and pointed his blue Mitsubishi toward a commuter parking lot near his home in Lake Ridge, Va. In another part of this Washington, D.C., suburb, Mildred Bowen put food out for her cat, packed a lunch, and, grabbing purse and briefcase, left her house.

Colonel LeBlanc and Ms. Bowen had never met, but within minutes of parking in the commuter lot, Bowen and another stranger were climbing into LeBlanc's Mitsubishi and driving off together.

This is not the first time Bowen has hopped into a car with total strangers - she has done this virtually every workday since 1995 as part of an ingenious commuting system that the Virginia Department of Transportation (DOT) says ferries an average of 6,500 people a day.

It originated during the gas crunch in the early 1970s, when carpoolers in the northern Virginia suburbs unexpectedly found themselves short a passenger. Cruising past a bus stop, they would offer anyone waiting there a free ride in exchange for the extra body that would grant them access to the High Occupancy Vehicle (HOV) lanes. Bus drivers dubbed these pseudo-carpoolers "slugs," after the fake coins used to scam free bus rides.

The name stuck, and the quid pro quo proved such a win-win that morning slug-lines formed at pick-up locations in the suburbs. Similarly, slugs formed evening lines at intersections in downtown Washington as well as across the river at the Pentagon and various business hubs.

SMOOTH SAILING: Commuters become 'slugs' so they can hitch free rides in HOV lanes that move swiftly. The non-HOV lanes at right are choked on Washington's I-395. ANDY NELSON - STAFF

Today, riders save up to $12 in fares and parking fees and, along with the drivers, anywhere from 30 to 50 minutes each way compared with driving in the regular lanes or taking public transportation.

"So you have this system that moves thousands of people every day," LeBlanc enthuses as he drives past a river of twinkling brake lights in the choked lanes to his right. "Nobody is really in charge, and it's organized by the people who use it. Where else would you find that?"

Nowhere, it seems - though with gas becoming as valuable a commodity as time, it might behoove others to look to Virginia as a model.

To take hold, however, slugging requires three key conditions, says LeBlanc. He ought to know: LeBlanc wrote a research paper on slugging in 1997, authored and self-published a guide to slugging, and in 1999 launched a website - www.slug-lines.com - that offers information and discussion forums. Slugs, he says, need a handy and free commuter parking lot; a backup mode of transportation, usually a nearby subway, bus, or commuter train stop; and, crucially, a carpool lane separate from regular lanes (the better to be policed) that requires three occupants per car.

HOV lanes requiring only two people don't do the trick. Finding one rider isn't that difficult and, as Bowen points out, there is safety in numbers. Settled in the backseat of LeBlanc's car, briefcase nestled at her feet, she says she "would be far less likely to hop into a car with one person unless I knew him."

Even with the security of a fellow slug, female commuters are cautious. "When I started," Bowen recalls, "if there were two men in the car that I'd never ridden with before, I wouldn't get in. Then you get used to seeing the same cars and the same people, and you get more comfortable."

The same applies to neophyte drivers. "When I lived in D.C.," says Caitlin Mackintosh, a slim blonde in the afternoon slug-line near a bus stop in Crystal City, "I used to pick up slugs and drive out to my karate class in Virginia. A friend came with me the first few times." She, too, grew so comfortable with the process that when she moved to Virginia, she chose an area with slug-lines.

Few slugs are ever stranded. "In 26 years," says Stewart Deavers as he waits near the Washington Monument for a ride home, "I've been stuck three times. Once I shared a cab back with this lady," he adds, pointing to a woman ahead of him in the line. Like most slugs, they know each other by face, not by name. She nods in agreement but doesn't engage; she stares instead at the stream of traffic, alert for the telltale slowdown that marks a driver in search of slugs.

In a city where the police chief recently declared a crime emergency, slugging has proved remarkably safe in its 30-plus-year history, thanks in great part to its home-grown rules of etiquette. If it gets dark, for example, slugs never leave a woman alone on the slug-line, and if slugs don't like the look of a car or its driver, they can pass.

Mostly, however, slugs "pass" because a car lacks air-conditioning or looks, as LeBlanc puts it, "kind of shaky," or is uncomfortable. "There's a guy that pulls up in a [Mazda] RX-7," LeBlanc says, eliciting a laugh of recognition from Bowen. He describes the sports car's backseat as "unfit for man or beast. You have to sit like this - " simultaneously, he and Bowen cock heads to one side.

Most of the guidelines deal with courtesy - slugs leave radio and temperature controls to the driver; they don't chat endlessly on their cellphones; and if the driver stays silent, so do the slugs. Should the driver give the go-ahead to chit-chat, conversation needs to steer clear of sex, religion, and politics. This doesn't mean that slugs don't exchange useful information. Mr. Deavers recalls a ride where the driver spoke of looking for a new office manager. By commute's end, Deavers's fellow slug was on her way to a job.

And politics do get discussed when the issues affect slugging. Large numbers of hybrid cars are exempt from the HOV's three-person requirement, and the DOT plans to build an additional lane in 2008 to accommodate single-occupancy toll-paying cars - "HOT." It's an acronym that perfectly describes slugs' reaction to both policies. The DOT says it will keep the number of HOT cars in check with toll hikes, but many slugs aren't buying it. "That makes sense in theory," says Ms. Mackintosh, "but the way people spend money here, they'd pay even higher tolls for the privilege."

She pauses while a Cadillac pulls up - leather interior, satellite radio. "I scored," she says, smiling. Minutes later, speeding home in luxury, Mackintosh and the driver say they will vote against HOT because the additional cars will eventually clog the HOV lanes. Take away the advantage of speed, and slugging could become obsolete. Until that sad day, slugging will continue to thrive. Although gas prices locally are as high as $3.29 a gallon, nobody is thinking of making slugs pay to ride.