A combination of apparent commuter frustration with Prince William County's congested roads and an energetic promotion of the county's ride-sharing program have resulted in an increase of more than 600 applications for rides since June, according to program coordinator Lauretta Ruest.

In an attempt to eliminate some of the congestion on Interstates 95 and 66 as well as the county's own snarled Rte. 234, Ruest has placed posters informing residents about the program in dozens of convenience stores, in county and state offices and in libraries. The posters, on cardboard easels, hold small boxes containing application blanks, which are being filled out at the rate of more than 200 a month since the promotion began.

Until the promotion began, Ruest said, ride sharing was one of Prince William's best-kept secrets. There were 400 riders and drivers in the computer's data base when she came to work for the county in late 1983. "Most people didn't even know we existed," she said.

Persons needing a ride to work in other parts of Prince William County, Northern Virginia, the District or suburban Maryland either come into the ride-sharing office at 9258 Lee Ave. in Manassas or call 369-POOL. A form that includes name, employer, work hours and preferred pickup points is filled out. That information is punched into a computer that is linked with the Council of Government's ride-sharing program. The computer then spews out a sheet of paper welcoming the prospective rider or driver to the program and listing several suggested "match-up" names to be called.

The sheet is then mailed to the person who made the request, and often that is the last Ruest hears of them. "Apparently," she said, "most people are satisfied with the service and the match-ups." Fees charged by the van or car drivers average $80 a month, an amount Ruest considers minimal, compared to parking fees and gasoline costs incurred when a person drives to work alone. Because a full car or van can use Virginia's High Occupancy Vehicle (HOV) lanes, the trip is usually 30 minutes faster as well, she said.

Prince William is the only Northern Virginia county where ride-sharing programs reach out directly to the community rather than operating through employers. Ruest said that Prince Williams's relatively low industrial and business base make her approach necessary. "That may change as the county grows," she said.

To encourage more van drivers to enter the program, the county offers low-interest, short-term $1,000 loans to cover a down payment on a van and other start-up costs. The borrower will not be asked to begin payments for three months after the loan is approved and has nine months to pay it back. According to Ruest, $9,000 is currently languishing in the bank waiting for future drivers to apply.

The program attempts to pick up a rider within a mile from home and drop him or her off within a mile of their job. There are six commuter parking lots throughout the county and several "informal" shopping mall parking lot pickup points that are used with the approval of the mall managers. Plans for one more commuter lot, financed like the others with state and local funds, are under way, Ruest said.

Although there are few residents currently using the program to get from one place in the county to another, Ruest's department is developing a ride-sharing plan for the 350 county employes who will be moved when the new county administration complex opens on Davis Ford Road in the spring.