The Fairfax County Board of Supervisors unanimously endorsed last night a route for the proposed cross-county Springfield Bypass that would stick largely to existing roads, have a 45 mph speed limit, and rarely exceed four lanes.

The board voted 9 to 0 to support a route, which, with only minor exceptions is similar to one first proposed in 1975. The 35-mile highway, which is expected to cost $200 million and faces uncertain funding, now goes to the Virginia Department of Highways and Transportation for final action.

The state is expected to decide by Aug. 20 where -- and whether -- to build the road, which would be one of the largest highway projects undertaken in recent years in the Virginia suburbs. If all goes as planned, construction could start by 1984, and the highway could be finished in another 10 years.

The Springfield Bypass -- it has been alternatively called the Fairfax Parkway and the Reston Bypass -- would transverse Fairfax County from north to south much like an outer loop of the Capital Beltway. It would run from the western edge of Reston in the north to Mt. Vernon in the south, providing better cross-county transportation than existing highways and improved access between the major Fairfax growth centers.

The road design approved by the supervisors yesterday has fewer interchanges than proposed by the state in an environmental impact statement released in April, and features more simple intersections, tighter curves, and traffic lights.

"This is an environmentally sensitive plan," said County Board Chairman John F. Herrity. "It sticks primarily to existing roads, it had a 45-mph speed limit, it's a parkway. We're really serious about this."

The Board also recommended that the number of six-lane sections of the road be kept to a minimum. They would be built in sections where traffic volume would be highest, such as near the access to the Shirley Highway (I-95) and the interchange with I-66.

The proposed road would originate in the north at Rte. 7, Leesburg Pike, and snake south between Reston and Herndon, intersecting Barron-Cameron Avenue. Slanting slightly to the southwest, it would cross the Dulles Access Road and then curl around the western rim of Reston, crossing Fox Mill Road.

At West Ox Cluster and Burchlawn, the road would veer to the southeast, then angle across routes 50 and I-66. At Rte. 29-211, it would make a turn to the east, swerving to the eastern side of the proposed county government center and then continue across Braddock Road and Rte. 123.

Heading almost directly to the east for about 10 miles, it would intersect Old Keene Mill Road, I-95, and Telegraph Road.

The road also provides access to the proposed Springfield-Franconia Metro station before feeding into Rte. 1 at its southernmost point.

Shiva K. Pant, Fairfax director of transportation, said after the vote last night that the board's modifications of the road were "designed not just to save money, but to minimize the adverse impact of the road upon communities."

The board recommended that the road have seven interchanges, down from the 13 to 17 proposed in the impact statement. Pant said that paring down the number of interchanges could save tens of millions of dollars.

"The design standards they [the design engineers] used were definitely at the upper end of the spectrum," Pant said. "One of the major things we're recommending is that the design standards be lowered."

Pant said the road will resemble the four-lane sections of Old Keene Mill Road and Barron-Cameron Avenue, both of which it intersects.

The board also requested that the Virginia highway department classify the road as a parkway.

One issue that may come back to haunt the board is the access to I-95. The state environmental impact statement had motorists taking quite a circuitous route from the road to I-95. The board's recommended route features a much more direct route onto the interstate, which runs the length of the east coast.

The final vote on the route did not come until about 7:30 p.m. The board originally was scheduled to consider the issue in the morning, but voted, 5 to 4, to put off consideration until after a public hearing July 25.

Then, in the early afternoon, the board learned that the state highway department refused to grant an extension of the June 30 deadline for the county's recommendations.