To thousands of motorists below on I-66, it is a brief moment in an unfinished tunnel where naked concrete girders stretch overhead.

To the scores of office workers and motorists above at Rosslyn Circle, it is yet another traffic obstacle on the rapidly changing face of Rosslyn.

In the future, Arlington officials envision the area above the highway as becoming an oasis for hordes of people during the day, a place to loll on the grass or eat their lunch on benches near a waterfall while a mime troupe or strolling violinists entertain them.

That scene would be the fruition of a $14 million plan to build Rosslyn Plaza Park on what amounts to an elaborately landscaped three-acre top for the I-66 underpass through Rosslyn. But unless Virginia highway officials who had initially resisted the park proposal change their schedules, those plans may be many years in the making.

Any work is not likely to be seen before early 1987, according to a Virginia highway official, who says construction money is tighter than rush-hour traffic on the North Lynn Street lanes leading to the highway.

P.L. (Buck) Veasey, supervisor of the state's highway engineering program, said $3.5 million in federal and state money earmarked for the park already has been spent, and the additional funds, including $160,000 from Arlington, are going to have to come from gasoline tax receipts.

Although County Board Chairman Ellen M. Bozman recently wrote the state Highway Commission urging that the project be completed at "the earliest possible date," Veasey said it is unlikely that the state will even ask for bids for another two years.

The park plan is the result of extensive negotiations between the county and the state highway planners. "Under the original plan, I-66 could have been just an open canyon there, a big trough," said Carlton Abbott, whose Williamsburg architectural firm is the park's design consultant.

"The county proposed the park when we were reviewing I-66 plans," said John Hummel, the county's engineering chief, "and suggested the area should be a real gateway into Virginia--an area that we would not want to have a open tunnel running through."

The state, Hummel said, finally agreed to provide the park, which the county will maintain. Hummel said the county has been reviewing the state plans for the park and will suggest only minor revisions when it responds this week.

The park will be bordered by the eastbound and westbound lanes of Lee Highway and North Nash and North Lynn streets. It will be divided at ground level by Fort Myer Drive, but plans call for a 40-foot-wide deck over the street, with ramps connecting the two sides of the park to the deck and to a Rosslyn skywalk. All of the park, including its deck, will be accessible to the handicapped.

Plans call for more than 150 large trees, such as flowering cherries, maples, honey locusts and lindens, more than 3,000 juniper, yew and holly shrubs and more than 12,000 ground-cover plants, mostly ivy, Hummel said.

In addition, there will be three major fountains, including one designed as a horseshoe-shaped waterfall, almost a quarter-mile of wall seats, 25 benches, drinking fountains, bike racks, rest rooms, five different kinds of lights and a 10-foot brick sidewalk around the park, Hummel said.

An unusual feature of the park will be an amphitheater recessed into the ground, in which architect Abbott said hundreds of people could be entertained. The design also includes small areas where kiosks or concession stands could be located, Abbott said.

"It's a new experience for us. . . . We certainly hope we're going to be able to experience some of the things there that we're already doing in urban plazas," said Connie McAdam, the county's recreation division chief. She wants the area to be a vibrant, people-oriented mall with art exhibits, concerts, lectures and festivals.

She concedes that noise from airplanes taking off and landing at National Airport will make it difficult to have regular concerts.

Because this is a park on top of the highway, Abbott said, there will be carefully hidden air vents to the tunnel beneath.

Asked if the noise from car and airplane traffic might not be a deterrent to spending an afternoon in the park, Hummel said: "I don't know. People sit on E Street and K Street just across the Potomac in downtown Washington within five feet of the traffic and eat."