The Metro subway system, reaching far into the Virginia suburbs, will open a 9.1-mile Orange Line extension to Vienna Saturday along the congested, rapidly developing I-66 corridor, offering rail service to thousands of commuters.

The $240 million extension, which includes four skylight-roofed stations in the highway's median, has been touted as a milestone in the expansion of Northern Virginia's transportation network and a long-needed addition to I-66, one of the Washington area's most intensively studied and hotly debated roads.

The extension should "substantially reduce commuting time" for many residents and allow them to "avoid the I-66 gridlock," said Vienna Mayor Charles A. Robinson Jr., adding that he expects to save about 15 minutes each way on his own ride to work in downtown Washington.

The new stations also have fueled controversies and raised prospects of intense development in some neighborhoods. Officials have predicted parking shortages and traffic problems. Homeowners have expressed dismay over mounting congestion and noise. A proposal for a privately built rail link between the Orange Line and Dulles International Airport has sparked dissent.

The areas surrounding the stations -- at East and West Falls Church, Dunn Loring and Vienna -- have become battlegrounds for developers and their critics. The sharpest clash occurred near Vienna, where developers planning a massive office center recently were dealt a setback by the Fairfax County Board of Supervisors.

Near West Falls Church and Dunn Loring, current plans call for moderate growth. Some officials say development may intensify if Falls Church annexes a prime tract of Fairfax County land next to the West Falls Church station. Homeowners near East Falls Church, however, are seeking to stave off development and preserve their community.

For Metro, the Orange Line extension is expected to be the last section of the long-planned rail system to be completed for at least four years. When the extension opens, the system will encompass 69.6 miles and 61 stations, more than two-thirds of the 103-mile rail network envisioned by local officials.

Not until 1990 are more stations scheduled to open. If sufficient funds are available, Metro plans to complete three sections that year -- a Yellow Line spur to Alexandria's West End, a Red Line extension to Wheaton and a Green Line segment connecting Anacostia with Northwest Washington.

One other Metro branch remains to be built in Virginia, a final section of the Yellow Line linking Alexandria's Van Dorn Street station with a Franconia-Springfield terminus. The Franconia-Springfield station, though still unfunded, has been targeted for 1994.

From Vienna, the Orange Line stretches 26.2 miles through the District to its Prince George's County terminus at New Carrollton. Riders who get on at the new stations can transfer to the Blue Line at Rosslyn to reach the Pentagon, Crystal City and other Virginia stops.

A rush-hour ride from Vienna to the Metro Center station in downtown Washington will take 30 minutes and cost $2.30, Metro officials say. At times other than rush hour, the fare will be $1.10. A 26-minute trip from West Falls Church to the Pentagon will cost $1.60 at rush hour and $1 at other times.

Trains serving the new stations are scheduled to operate every six minutes at rush hour and every 12 minutes during nonrush-hour periods, including evenings and weekends.

To reduce crowding, the transit authority recently increased the size of trains on the Orange Line from four to six cars. If crowding occurs after the new stations open, officials said, additional trains will be dispatched during peak periods, probably around 7:30 a.m. and 5:30 p.m.

Surrounded by parking lots with more than 4,600 spaces, the new stations are expected to draw thousands of commuters from Arlington, Fairfax and Prince William counties and from Falls Church, Fairfax City and Manassas.

Many riders are expected to take buses to the stations to avoid prospects of jammed lots.

24,000 Riders a Day Expected

Metro officials have predicted that the number of one-way trips taken by passengers using the new stations will climb to 24,000 a day by the fall. Total ridership on the Orange Line is expected to rise less sharply because some commuters who now use stations in Arlington may switch to the new stops.

For the most part, officials said, the stations, electronic rail equipment, parking lots and connecting roadways are ready.

The Metro system, long troubled by shortages of subway cars, says it has an adequate fleet. In a display of confidence, Metro agreed to open the stations a month earlier than once planned.

"We feel pretty good about it," said Fady P. Bassily, Metro's assistant general manager for rail service. "Everything is on schedule."

Some key problems remain. Improvements to access roads near several stations have been delayed, drawing complaints from local officials. Work also has lagged on safety barriers being installed along the I-66 median to reduce risks of collisions between trains and cars or trucks.

The improved barriers and other safety measures were planned after a 1983 accident in which a car went out of control, crossed an existing barrier and landed on Metro's unopened tracks. A double row of ordinary barriers will be set up until the new ones are completed, officials said. Other safety devices, including an electrical alarm system, have been installed.

Nowhere else in the Metro system has a rail line been built in a highway median, and the issue has repeatedly caused concern among local and federal safety officials. Metro has taken other steps to eliminate traffic hazards on I-66, including reducing glare from a skylight and shielding train signals.

The new stations are scheduled to open at 8 a.m. Saturday, with free one-way rides until 4 p.m. Ceremonies are planned at all four stations, starting at 9 a.m. at East Falls Church.

The extension's opening will mark a final episode in one of Virginia's most hotly contested transportation battles, a protracted conflict between highway advocates and environmental activists over proposals for completing the last stretch of I-66 inside the Capital Beltway.

In a key 1977 ruling, U.S. Secretary of Transportation William T. Coleman Jr. approved the existing scaled-back version of the I-66 segment but, at the same time, required Virginia to allot millions of dollars to help build the Orange Line. State officials previously had balked at providing the funds.

For state Sen. John W. Russell (R-Fairfax), the extension's opening will represent a different sort of occasion. In a widely noted show of skepticism in 1971, Russell, then mayor of Fairfax City, predicted that he would "see God before I see Metro." He later promised to eat the text of a story quoting his remark in the old Virginia Sentinel if the extension ever opens.

"I hope I don't have to eat it," Russell said a few days ago. "I'm dubious until June 7."

Recently, parking has become a key issue. Metro's lots at East and West Falls Church and Vienna are expected to fill up regularly on weekdays within a few months after the stations open.

Spaces may be available for some time at the Dunn Loring lot, officials said, but the lot cannot be reached directly from I-66.

County and city governments have taken steps to keep commuters from parking in residential neighborhoods near the stations. The town of Vienna is prepared to issue tickets carrying possible $50 fines for parking illegally on streets reserved for cars with residential permits.

Metro and other local agencies plan major shifts in bus routes to serve the new rail stations. Starting June 22, many Metrobus routes will be revised and service will begin on several new routes with stops at the stations. However, bus service on other routes that use I-66 will be halted to cut costs.

Some Metrobus fares will be modified. Zone crossing fees will be eliminated on routes near the new stations, and express surcharges on some Reston routes will be dropped.

Although most Metrobus routes serving Reston will be revised to terminate at the West Falls Church station, Metro plans to retain several bus routes connecting Reston with the Pentagon and Crystal City. Passengers complained that eliminating those routes would create inconvenient transfers.

Bus System Expansion Planned

The Fairfax City government plans a major expansion of its bus system, called CUE, to serve the Vienna station. The system, jointly managed with George Mason University, will offer free rides from Saturday until June 30. The normal fare is 25 cents.

The Northern Virginia Transportation Commission plans to operate a 14-passenger van, called the Tysons Shuttle, between the West Falls Church station and the Tysons Corner area, starting June 16. Vans will run every 15 minutes on weekdays from 6:30 to 9:30 a.m. and from 4 to 6:45 p.m. The fare will be 60 cents, with discounts for multiple-trip tickets.

The Washington Flyer is scheduled to provide bus service between Dulles International Airport and the West Falls Church station daily from 6 a.m. to 10:20 p.m., starting June 22. Buses will run every 45 minutes, officials said. Fares for the approximately 17-minute ride will be $7 one way and $13 for a round trip.

Prince William County officials said they plan to consider starting bus service between the Manassas area and the Vienna station on the county's Commuteride system in several months.

Fairfax County officials have begun weighing a possible expansion of the county-run Fairfax Connector bus system to serve the Orange Line stations. In Loudoun County, officials said they may eventually consider providing bus service between the Sterling area, north of Dulles, and the West Falls Church station.

The West Falls Church station has been cited as posing the most severe access problems among the new Metro stops because of delays in a controversial road improvement project. Access roads also are at issue at Dunn Loring and Vienna.

At West Falls Church, the complaints center mainly on Haycock Road, a hilly two-lane street regarded by many officials as hazardous. No walkway has been built for pedestrians, and other improvements have been delayed by a protracted debate over the project among Fairfax, Falls Church and state highway officials.

A temporary walkway along part of the road is expected to be constructed this summer, according to county officials, but plans to widen and level out the road are not expected to be completed for three years. Meanwhile, left turns from Haycock Road to the station have been prohibited.

An alternative access road to the West Falls Church station also may cause confusion for commuters, officials warned. The access road branches off a ramp leading from Leesburg Pike (Rte. 7) to eastbound I-66. Officials said that car pool restrictions normally in effect on I-66 will not be enforced on the ramp.

Access to the Dunn Loring station will be hampered, officials said, because a planned extension of Prosperity Avenue has not been built. The road extension, designed as an alternative to the existing access route from heavily used Gallows Road, is expected to open next year.

Near the Vienna station, two uncompleted highway projects have caused concern. A plan to widen and improve Blake Lane is scheduled to be carried out by 1988, officials said. A new $17.7 million interchange at Nutley Street and I-66 is considered likely to open in 1989.