When Colonial Transit Co., a commuter bus firm, faltered in 1979, Prince William County officials rushed to commission a study by a top accounting firm outlining ways the county could step in and provide commuter service.

Three weeks ago, when the same company, under different management filed for protection in federal bankruptcy court, county officials were less than enthusiastic about shoulderf transporting its commuters into the District.

"We have a keen interest in keeping this in the private secble," County Executive Robert Noe said last week.

County officials have always maintained that transportatip priorities. But in the last few years, they say they've decided the county cannot afford to pay for commuterInstead, the county has aggressively gone after state and federal grants to aid the private sector in providing mass transportation.

Prinlliam, with 144,000 residents, is growing at a rapid rate as people give up proximity to the District in favor of less expensive housing outside the Capital Beltway. Transportation from Prince William into the District, 30 miles n limited to private cars, van pools and a handful of private bus routes.

The result is 45,000 commuters clotwo interstate highways each day, creating traffic jams and pollution.

Prince William's commuter problems are typical of many communities in the outlying districts of cities throughout the United States, said Stephen T. Roberts, acting executive director of the Northern Virginia Transportation Commission.

In its search for a solution to the problem, the county has toyed with the idea of asking the Washington Metropolitan Area Tranhority to extend its Metrobus routes into Prince William.

But such a move would cost the county an estimated $7 million a year in bus subsidies, said Roberts. And the cost of extending the Metrorail line from Alexandria up the Potomac River to Prince William would be "out of this world," he said.

"If we live twice our life expectancies, we wouldn't see Metrorailam," said Noe. "It is a fiscal impossibility."

Yet it is difficult for private commuter bus companies in th operating, Roberts said. The cost of buying buses and running them into the District is often higher than the be found in the fare box.

"County commuters will only pay so much for a ticket into D.C.," he said. "If the price is too high, they will hassle the traffic and drive themselves or join a car pool."

Colonial Transit once serviced 3,000 commuters, but riders say as fares increased and quality of service slackened, more and more residents turned to other means of transportation. The company was down to 900 customers when it filed for protection under federal bankruptcy laws July 1.

Noe said the county plans to apply for more and more federal and state grants to buy buses and then lease them to private operators. Last month the county announced it had received its first such grant, $1.4 million from the Virginia Department of Highways and Transportation with whichns to buy about 20 renovated buses and lease them.

"It's a good plan," said Roberts last week. "It is definter community can provide bus service at a nominal cost."

Roberts said the cost of buses is what often stands between a private bus operaprofit. If the county, using state grants, could purchase the buses, then the operators could make a profit frand from the District and charging reasonable fares.

"Places like Prince William have to be creative when isaid. "Using private and public money together could be the best way to go."