TWENTY YEARS of federal exclusiveness really ended last week when Transportation Secretary Brock Adams decided to let commuter car pools use the Dulles Airport access road. It's about time. The airport traffic for which the 13-mile, four-lane drive was built has not materialized and is not now expected until after 1985. Meanwhile, everyday traffic has piled up on the ordinary roads nearby, making the Dulles road's emptiness look more arrogant by the day.

The airport's guardians in the Federal Aviation Administration have feared - with reason - that if all bars were dropped, the drive would be overrun by commuters and rush-hour access to the airport would become impossible. The limited plan endorsed by Mr. Adams last week avoids that jam. It would simply enlarge the access ramps at Reston, now used by buses, and open them to cars with four or more occupants during rush hours. No new access points would be built. And by 1985, when Interstate 66 has been opened inside the Beltway, the commuters would be shifted there.

This sensible approach should encourage car-pooling and help ease the strains on Route 7 and other nearby roads. DOT's traffic estimates - about 840 car pools a day by 1984 - were made before the gas crisis and are almost certainly low. Even so, the road should not be overwhelmed - if access is carefully policed. Mr. Adams wants the Virginia highway department to take on the enforcement job. That is appropriate, and would provide a good test of some of the rush-hour controls that will be needed on a larger scale along 166.

Beyond these immediate gains and challenges, of course, are the longer-range problems of transportation in the fast-growing Dulles corridor. While welcoming the car-pool plan, Fairfax County officials have been quick to emphasize that they still want to build a toll road beside the Dulles lanes to serve local traffic and stimulate business development.

Though the toll approach is novel for Northern Virginia, the federal plans and right-of-way to Dulles have always left room for parallel local lanes. Yet this 20-year-old policy should be reexamined, too. The energy crisis makes it imperative for suburbs and new employment centers to develop more alternatives to travel by car. Extensions of Metro, or connecting streetcars or trains, might be more farsighted forms of local transportation than other road. Certainly any highway plan should put heavy emphasis on buses. And whatever happened to the idea of Metro service to Dulles? Twenty years from now, running Metro even farther - perhaps to Leesburg - may be very desirable.

Owning the airport and the right-of-way gives DOT powerful leverage and a special responsibility. Before yielding any ground to the highway-builders, DOT should require the state and county to join in a review of transportation policies for the airport and the Dulles area.