The proposed route and design for the cross-county Springfield Bypass approved Monday night by the Fairfax County Board of Supervisors strike a balance among economic, environmental, and transportation interests. But how much sway the board's recommendations have -- and whether the 35-mile, $200 million highway will be built at all -- remains to be seen.

The project, which now goes to the Virginia Department of Highways and Transportation for action Aug. 20, would, county officials say, be one of the most significant items on the Fairfax transportation agenda in the 1980s. To hear some county officials tell it, the highway could be more important to Fairfax than the long awaited opening of the first Metro station there in late 1982.

"This road is the last shot we have -- the last hurrah," said Board Chairman John F. Herrity, who has long championed the road-building needs of the county. "If this didn't fly, nothing ever would."

Using much improved existing roads, by and large, the Springfield Bypass would sweep from Rte. 7 near Herndon and Reston in the north to Mount Vernon and Fort Belvoir in the south, intersecting key roadways, easing traffic jams, and in segments spurring the county's economic development.

At first blush, the route seems difficult to fault. Transportation planners danced a political and diplomatic minuet in plotting the road, and it seems that they stepped on few toes.

The road, because some 15 miles of it would be a simple expansion of roads already on the map, and because the county is recommending that only seven interchanges be built, would not be extremely expensive by modern-day road-building standards.

With the recommended four lanes (six lanes would be built only in areas with the highest anticipated congestion) and 45 mph speed limit, the board also hoped to build the most attractive road possible. "This might not be another George Washington Parkway," one county staffer said yesterday, referring to the picturesque roadway that runs along the Potomac, "but it certainly won't be another Shirely Highway [I-95 and I-395]."

The highway would not break up a single existing neighborhood, according to county transportation director Shiva K. Pant, and fewer than 100 single-family homes would be displaced by the time the road is finished -- perhaps 1994.

Moreover, Fairfax developers such as John T. (Til) Hazel would be well served by the road, which will provide dramatically improved access to the largely undeveloped 2,500 acres of land around the proposed county center west of Fairfax City. It would boost property values along the road throughout the county by as much as $256 million, county officials say.

As a boon to the developers, the road would also coincide with the board's long-range goals for Fairfax, which call for a broader tax base through greater development.

Finally, there seems little doubt that the road, if it is adopted by the state, would accomplish Fairfax's professed foremost goal: to ameliorate cross-country transportation, which has become increasingly inadequate as Fairfax has expanded over the past decade.

"If you've ever tried to drive from Herndon to Ft. Belvoir, you've found you can't do it -- or that it's awful hard to do," said Fairfax public affairs director Edmund Castillo. "But this road would change all that."

Fairfax transportation planners also have linked the project to Metro, stressing that the road would provide vital access to the Springfield Franconia Metro stations. Without the road, according to Pant, there would be no easy way for commuters to reach the stations, scheduled to open in 1990.

But roads, like diamonds, are both forever and expensive. And there is no clear indication that the needed state and federal funds are available to build it.

Supposedly, as much as 75 percent of the cost of construction would be paid by the Federal Highway Administration. But funds of the type that have been applied to roads such as this one are under review by the Reagan administration and may be phased out in large part by 1984.

That would leave the financially strapped Virginia highways department to pick up at least a chunk of the tab, which probably would not be possible.

Some county officials were confident, however, that funds for the road will be found. Said board chairman Herrity: "I'm not sure all of [the by pass] will ever be built, but, yes, I think there will be money from the federal government."

"We're going to find the money, somehow," said William B. Wrench, Northern Virginia's representative on the state highway commission.

Another question that looms large before the county is how closely the state will adhere to the county plan for the road. An environmental impact statement prepared for the state by a private consulting firm recommended a significantly more extensive road, in terms of number of interchanges and number of lanes, than the county has in mind.

"The design of the road could change the whole character of the community it goes through," said board vice chairman Martha V. Pennino of the Centreville District in an interview yesterday. "We have got to continually monitor to make sure that we don't have another Shirley Highway going through the middle of the county."