Steve Carapetis walks to a Metro bus stop near his suburban Virginia home every work-day morning, headed for his World Bank office in downtown Washington.

Yet, to the chagrin of transit officials, only once in the last 18 months has Carapetis hopped aboard a Metro bus.

Instead, he has joined a growing number of Virginia commuters who are shunning Metro for their own underground transit system, one that comes cost-free to the riders and avoids many of the bus system's headaches.

The underground system is created each work-day morning by hundreds of motorists who cruise Metro's bus stops, more than willing to offer free rides into the city in return for securing enough passengers to qualify for the Shirley Highway's express lanes. The lanes, open to buses and cars with at least four riders, can cut 35 minutes off a rush-hour trip into the District and bypass eight miles of bumper-to-bumper traffic.

"This is fantastic," said Carapetis the other morning as he waved for a ride near his Fairfax County home. "There's really nothing like it anywhere in the world."

While it's difficult for area officials to assess the impact of the impromptu car poolers, some Metro transit officials are troubled by their operations. "There is no doubt that these people are skimming the cream off our service," says Metro planner Bob Pickett.

Because of them the transit system is suffering "a direct loss in revenue," Pickett says, a deficit that some officials have estimated could be as much as $250,000 a year. Pickett fears their impact may force Metro to make bus cutbacks along the Shirley Highway, cooridor, once considered the nation's premier suburban bus route.

Still worse for Metro are the troublesome aftereffects of the instant carpools. Once lured away from the bus, many of the hitchhikers say they aren't about to return to Metro.

"The bus service is deteriorating," complains Carapetis, who says he can slice at least 30 minutes off his morning commute by riding with anyone, but Metro. "The bus takes too long, it's too inconvenient, and the change from bus to subway at the Pentagon just adds more time."

Other Shirley Highway hitchhikers say that it's just more pleasant to ride in an automobile, Metro "was very stressful," says Judith Warner, who once considered quitting her job at a Washington law firm rather than continue riding the bus. "If you're packed into a subway car like a sardine or standing on a bus for 45 minutes, all you want to do when you get home is relax."

She is not alone. A recent morning found a crowd of affluent, well-groomed government workers at a Springfield bus stop, each haggling with passing motorists about their destinations in the District and each seeking a car that would get them closer to their office. Among them were an aide to a senator, a State Department official, a speechwriter and a labor relations analyst.

None expressed fears about riding with strangers. "I'm not really worried about running into anybody obnoxious," said Allingham, a legal secretary who estimates she has saved hundreds of dollars by picking up rides to the city. "I can't imagine anything bad might happen. How can they abduct you when there are four people in the car?"

Suburban police say they known of no crimes linked to the instant car pools, but they tell of angry businessmen who fume at suburbanites who jam parking lots near the bus stops as they barter over rides.

"I went out and talked to them (the riders) and they fumes Dr. Anthony got belligerent," Viscidi, a Springfield dentist who has posted "No Trespassing" signs on a parking lot at his Keene Mill Road office that has become one of the most popular meeting spots. "I feel like I can't do anything about it unless I hire some big 6-foot-2 thug to stand out there for me," the dentist says.

Fairfax Supervisor Marie B. Travesky (R-Springfield) says she is trying to get a new collection point for the commuters, but county police say Viscidi will have to fend for himself until that can be arranged.

"As far as traffic ordinances go, they're not doing anything illegal," said Fairfax County police spokesman Warren Carmichael. Maryland officials say they have not seen the same phenomenon because none of Maryland's major roads into Washington have express lane's similar to those in Virginia.

California officials have given official sanction to a similar system in Marin County, across the Golden Gate Bridge from San Francisco. But because of the fears of crime, drivers participating in the program there are required to be registered and photographed as a security precaution.

Almost 1,200 motorists have signed up for the program, which is called the Marin Commuter Connection and is being copied by Prince William County officials in Virginia.

With increasing numbers of motorists seeking to use the express lanes, some of the Virginia hitchhikers say they now can afford to become picky about their rides.

Phil Bohall, who commutes from Springfield to the Postal Service Headquarters at L'Enfant Plaza, says he will only ride with people who speak English. "And I won't get in a junk car. You've got to protect yourself, you know."

Other riders say they attempt to weed out speeders, tailgaters, and drivers who talk too much, but few wait longer than 10 minutes for an acceptable ride and all express satisfaction at their success in beating the traffic on the region's most crowded highway.

"I love it -- it's convenient, fast, and economical," says Gail Lockhart who rides to the Department of Energy. "I'm helping someone else because they need to get into town fast, and they're helping me by giving me a free ride. Everybody wins."