Hitchhiking has become so popular in Fairfax County that commuters now can get rides without raising a thumb.

As many as 500 to 1,000 commuters a day, depending on the weather, are picked up along Springfield's Old Keene Mill Road by motorists seeking to create instant car pools so they can use the express car pool and bus lanes of Shirley Highway.

This free-enterprise transportation system, which sprang up beside the road in 1974 when the express lanes were first opened to four-person car pools, has caused so many traffic jams in recent months that Fairfax County police began ticketing motorists who stopped to pick up riders.

Now, at the request of some of their hitchhiking and motoring constituents, two supervisors have stepped in to help devise a better system.

And county officials are looking for ways to solve commuter-hitchhiking problems expected to develop this fall, when I-66 opens inside the Beltway. During rush hour, only car pools and buses will be permitted on the new section of I-66, and similar clusters of commuters are expected to appear at I-66 entrance ramps.

In Springfield, a new off-the-road system was worked out with help from commuters, Board of Supervisors Chairman John F. Herrity, Lee District Supervisor Joseph Alexander, and two Old Keene Mill Road businesses, Gino's and Springfield Cinema I & II.

The new system, with motorists swinging into the parking lots of Gino's and the theater to pick up riders, was working only fitfully earlier this week.

Commuters generally ignored the signs put up to designate waiting areas for "Memorial Bridge" and "14th Street Bridge" car pools and simply milled about the lots. The courting motorists entered and circled the lots, and the drivers called out their destinations like vendors at a farmers' market:

"State Department."

"14th and Constitution."

"Capitol Hill."

"20th and L."

It was still a hitchhikers' market, as it was out on the road. No commuter had to wait more than five minutes for a ride, and usually there was a choice of several groups.

Not all the motorists were as fortunate, however. Several impatiently drove off without acquiring the magic number of persons for a car pool, although in most cases they were heading for less popular destinations.

"It's great in the mornings," said Richard Frederick, who has been walking half a mile from his house to Old Keene Mill for almost four years to hitch a ride to the Treasury Department. "But in the evenings it doesn't always work. There's no one place to wait downtown, although I often can get picked up by waiting at the bus stop at 14th and Constitution."

Frederick took to the streets because "it takes three times as long and costs three times as much to ride Metro," he said. When Metro stopped its Springfield buses at the Pentagon and commuters were forced to transfer to the subway, "that's when I left the bus system," he said.

Alan McKie, a Rolling Valley resident who both drives a car pool and has hitched rides on Old Keene Mill, was one of the commuters who urged county officials to step in and organize the hitchhiking.

"For a while it was in front of a dentist's office, and when he complained we moved in front of a bank and then in front of Gino's. . . . We've been moved around but it's a marvelous system. It saves gas and everybody saves 20 minutes or so getting to work. Everyone benefits," said McKie, the deputy managing director of the Federal Communications Commission.

McKie called Gino's corporate headquarters to enlist support for parking lot pickups. He also called Herrity.

Another commuter who attended the hitchhiking summit conference in Herrity's office, Philip Bohall, is another former bus rider who says he joined the ranks along the road "when direct bus service to Washington stopped."

Like some of the Springfield hitchhikers, Bohall parks in a field across Old Keene Mill behind a bowling alley. Others park in the Cinema I & II parking lot by becoming "patrons" and buying $20 worth of movie tickets every three months--a daytime business that has noticeably increased theater patronage, says Cinema manager William Breedon. Breedon was out at dawn Monday, directing traffic in his lot. In January, bags of popcorn were given to "patrons" of the movie house.

Bohall did a survey last fall and found "there were about five major destinations: Farragut Square, 14th Street, L'Enfant Plaza, Foggy Bottom and the Pentagon."

He also found that almost as many women as men were hitchhiking, and that "most of the riders appeared to be over 30." Women hitchhikers interviewed this week all said they have had no problems riding with strangers. In fact, "this is a nice gathering," said Bobbie Krull, who has been picking up riders or hitching herself for three years. And while their fellow passengers may be strangers at first, Krull and other women said they have come to recognize many of the commuters, and have made new friends on the daily trips.

The majority of cars picking up commuters, said Bohall, are "regular car pools lacking a rider or couples who both work" downtown and find it cheaper to drive.

Bohall said that while the commuters "are a great cultural mix of people, most are three-piece-suit types. And most of the cars are new. People tend not to get into junkies," he said. Bohall said he hasn't been on Shirley Highway except in car pools for three years, and is not sure exactly how much time the car pool lanes save--probably 20 to 40 minutes or more if there's rain or an accident along the route, he said.

"But a lot of people like to drive by themselves and don't mind the traffic jams. They find solace being away from their families and the office, being alone on I-95 for an hour or so and listening to the radio," Bohall said.

Conversation in the instant carpools is usually relaxed and friendly, he said, "with people chatting about traffic and current issues. But if there's a sleet storm you tend not to bother the driver."

Bohall said he "got up a petition, with mail-in cards to Herrity" to help spur official action. But he cautioned that the new off-street pickup arrangement may take a while to begin working smoothly.