The Fairfax County Board of Supervisors yesterday became the first local governing body in the region to endorse the concept of an outer beltway, which would run from I-270 near Frederick, Md. to I-95 north of Fredericksburg, Va.

Although the road would bypass Fairfax to the west, county officials see it as as a way to divert north-south traffic that would otherwise use the Beltway and segments of I-95 within the county -- including many tractor-trailers that now use the Beltway and have accounted for three major accidents causing traffic tieups in the last 30 days.

The vote in favor of the outer beltway was 8 to 0. Supervisor Joseph Alexander (D-Lee) was absent.

Supervisor Thomas M. Davis III stressed that Fairfax must now rally support among the outer suburban counties in Virginia and Maryland, which the road would traverse, if the outer beltway proposal is to get off the ground.

"We're going to have to involve those other jurisdictions," said Davis, "because without their approval it's not going anywhere."

There is no definite route set for the new beltway, being called the outer beltway or Washington Area Bypass, nor has it yet been studied by the Virginia Department of Highways and Transportation. Essentially, though, it would sweep south from near Frederick, crossing the Potomac River east of Leesburg, according to officials. From there it would slice south through Loudoun County west of Goose Creek and through Prince William County east of Gainesville. It would then link up with I-95 in the south.

The supervisors, who last week assailed what they called lax regulation of truck safety and debated steps to tighten control of the 18-wheelers, renewed their discussion yesterday.

However, County Board Chairman John F. Herrity, a Republican, said, "The political realities are that we are not going to ban trucks from the Beltway."

Herrity added that closer inspections of the trucks could enhance their safety, and several supervisors said trucks should be governed by a 40 mile-per-hour speed limit and restricted to the right lane.

In other actions yesterday, the supervisors:

*Said they would seek a state law to give the county more leeway to have junked vehicles towed away.

*Authorized the purchase of 1.6 acres near Baileys Crossroads for the first of two planned county-operated shelters for the homeless. The second will be in the Mount Vernon area.

*Agreed to look into establishing a category of "high-risk" developers who would be required to post bonds in cash rather than through insurance firms or letters of credit because they have a history of defaults.

*Denied a motion by Supervisor Audrey Moore (D-Annandale) to cut sewer-user fees. Moore argued the fees should be reduced in keeping with a pledge by board members in July that taxpayers would not have to finance a sewer plant expansion. Other supervisors contended, however, that the portion of the sewer work to be financed by taxpayers is not related to the plant expansion.

The outer beltway is the second major proposed artery in Northern Virginia that would roughly parallel the existing Capital Beltway. The first, known as the Springfield Bypass, would cut across Fairfax County, connecting Leesburg Pike (Rte. 7) in the west with U.S. Rte. 1 and I-95 in the south. Funding for the first major segments of the Springfield Bypass is included in a $135 million bond referendum that will be put to county voters Nov. 5.