ANYONE WHO HAS SHOPPED at Tysons Corner can probably understand the Fairfax County Board of Supervisors' decisions to defer action on Tysons II, the huge new shopping center that developer Theodore N. Lerner wants to build across the street from the existing mall. They Tysons area, at the intersections of the Beltway, Route 7 and Route 123, is already renowed for good stores and bad traffic problems. Another million-square-foot mall would mean lots more of both.

In fact, the jams there need to be unsnarled whether or not anything more is built. Through traffic coule be moved out of the shopping area by building the long-discussed parallel lanes along the Dulles access road between Route 7 and the Beltway. However, that requires cooperation from the FAA, which owns the Dulles road, and also waits on a final decision about - you guessed it - I-66. Some lesser projects are in the works, but major measures will take years and must compete for state construction funds against every other congested intersection in Northern Virginia.

So this might seem to be a bad time to consider anything like Tysons II. However, the choices are very limited. In theory, further development could be curbed through downzoning. But the Virginia courts are not very hospitable to growth controls, so this is probably impossible. What if the board simply rejected the Tysons II rezoning request? Under existing zoning, the Lerner acreage could be developed any time with high-rise offices and apartments that would probably produce more rush-hour traffic than Tysons II. If no rezoning were required, the county would have little say about project design and no real way to persuade the developer to help finance adequate roads.

The Tysons II rezoning application gives the county considerable leverage - and the county is using it. The planning commission and a citizens' task force have put great emphasis on attractive design and landscaping and on finding ways to help customers get around within the shopping area. A key issue for the board of supervisors is how much Mr. Lerner is willing to contribute toward better roads. He has offered to contribute $1 million, but that may be too little, and his proposed 1980 deadline for starting work would obvioulsy be impossible for the highway-builders to meet. By November, when the board takes up the rezoning again, Mr. Lerner may well have decided to raise his offer and stretch out his timetable. If he wants the project badly enough to do that, and if the county and the state can settle on a sound and fairly far-sighted traffic-improvement plan, the results could be very good for both traffic and trade.