The Fairfax County Board of Supervisors, long angered at escalating Metro bus and rail subsidies, this week took the first step toward setting up a county-operated bus system.

Voting unanimously, the board asked county staff to study the fleasibility of setting up a feeder bus system that would serve the Huntington Metro station, which is to open in 1982.

The supervisors also called for preparation of draft legislation that would give the county authority to use its own funds for constructing and improving highways.

The two decisions come amid indications that the county's mounting traffic congestion problems are likely to get far worse before they get better. A staff report presented to the board this week noted that automobile traffic in the county is increasing by leaps and bounds, while state money earmarked for new highways is dwindling.

With a few exceptions, Virginia law prohibits county governments from building their own highways.

"Everyone's talking about Metrorail, but what we really have is a traffic problem out there -- and it gets worse every day," said county transportation director Shiva K. Pant.

The county estimates that highway traffic in Fairfax has increased almost twice as much as home construction over the last 10 years.

Officials attribute the rapid increase to development trends that place new homes farther from the Beltway, as well as to the county's tendency to attract wealthy residents and the growth in the number of commuters who pass through Fairfax County on their way to work.

Officials expect total transporation trips in the county each day to top 1.5 million by 1990.

At the same time, only five of 15 county road improvement projects planned by the state in 1973 has been completed by 1979. According to county figures, rush-hour traffic on most county figures, rush-hour traffic on most county commuting routes already exceeds accepted standards.

County officials originally had considered pulling out of Metro entirely, thinking that such a move would save them money in the face of rising Metro labor costs. But Pant aruged in a memo to the board this week that setting up a new bus system could cost county residents more than the present arrangement.

The board finally accepted Pant's suggestion to plan a smaller transportation project "in order to conduct a realistic study in a manageable cost and acceptable time frame." Pant stressed that the Huntington study was not meant to preclude future studies of broader county-operated bus systems.