The Fairfax County Board of Supervisors decided yesterday to let the Virginia Department of Highways and Transportation choose which proposal for a major suburban highway should be pursued.

The six-lane, $135 million highway, called the Springfield Bypass, would stretch 30 miles from the northwest to the southwest part of the county. Proponents say it would cure the area's long-congested road system, while opponents include those who fear it would stimulate development and actually increase congestion.

The board's resolution asks the highway department to consider a route proposed several years ago in a county master plan and the effects of not building the highway. Two other routes have been proposed to the state by a county citizens' committee and the department's own consultants.

The board voted 7 to 2, with Chairman John F. Herrity (R) and Supervisor Audrey Moore (D) opposed, to postpone until next year a hearing on the bypass to give the highway department time to decide which proposal is worthy of further study.

In another matter, the board deferred for two weeks debate on a sewer project that would benefit development plans of George Mason University and John T. Hazel Jr., a major county developer who sits on the unniversity's board.

The Washington Post reported yesterday that Hazel, who abstained from the university board's vote endorsing the project, has been lobbying county officials to support it. The project would increase development in an environmentally sensitive area that drains into the Occoquan Reservoir, a prime source of drinking water for Northern Virginia.

Moore proposed that the County board ask the state attorney general for an opinion on whether Hazel's lobbying constituted a conflict-of-interest, but her motion died for lack of a second.