When a 3.5-mile extension of the Dulles Airport Access Road opens in early November, it will give motorists a continuous expressway between Washington and Dulles International Airport that planners 30 years ago said was crucial to the airport's future.

The $25 million extension, which connects the existing 15-mile access road with the recently opened section of I-66 inside the Beltway, will cut travel time between Washington and the airport by 15 minutes, to about 25 minutes, and possibly shave as much as 30 minutes off during rush hour, Federal Aviation Administration officials predict.

Residents in Loudoun and western Fairfax counties will benefit from the new road as much as airport travelers because the FAA officially has opened the existing access road to four-person car pools in rush hour and is considering relaxing that restriction to allow fewer persons in the car pools.

Only four-person car pools can use the new extension during rush hour, however, because it has no exits and connects directly with I-66, which has a strictly enforced four-person car-pool restriction.

Extension of the Dulles Access Road, coupled with planned improvements to airport bus and limousine service in the spring, "should dramatically increase travel at Dulles," said Richard A. Griesbach, business manager for the federally owned Dulles and National airports.

The FAA is buying 14 buses and is contracting to provide a door-to-door limousine service for much of Northern Virginia and Montgomery County that would pick up airport travelers at their homes and take them to the buses. It is seeking contractors to operate this expanded airport ground-transportation system.

The extension will look much like the existing Dulles Access Road but will have sound walls and be much more heavily landscaped to screen the four-lane highway from the many residential developments along its route. However, it may look somewhat barren when it opens in early November--the exact date has yet to be set--because most of the landscaping is not scheduled to be planted until spring, said Frank J. Conlin, chief of the FAA's regional engineering staff.

While there are no entrance or exit ramps along the extension, the FAA has made provisions for special commuter bus ramps to be built in the future at Magarity Road (similar to the ramps from the existing access road to Reston Avenue) if Virginia pays for them. The state has no plans for such ramps, although it is building bus ramps from the access road extension into the West Falls Church Metro station on I-66, in time for its opening in 1985.

Like the existing access road, the extension will have a wide center median where a future Metrorail line can be built, connecting Dulles with the subway at West Falls Church. The airport buses will stop at the Metro station, giving airport travelers the option of transferring to the subway or taking airport limousines or taxis from the station.

The existing access road has been carrying about 38,000 cars daily, including at least 6,000 "backtrackers," commuters who drive to the airport where they turn around to take the access road to Washington.

Traffic on the access road is expected to rise to about 68,000 vehicles a day by the year 2000, without backtrackers, who will be banned from the airport road after the parallel Dulles toll roads are opened within the next two years.

Completion of the road also will solve a small problem for I-66 at its Beltway interchange, where there is only a partial interchange--motorists traveling outbound on I-66 cannot get onto the Beltway heading north. By getting onto the new access road extension they will be able to exit onto the Beltway's northbound lanes.

The FAA bought most of the land for the extension of the access road in the 1950s and early 1960s, but its construction always has been contingent upon completion of I-66 inside the Beltway, which was embroiled in court fights and controversy for more than 20 years before a scaled-down and restricted highway was approved in 1977 by federal transportation officials.

Then Secretary of Transportation William Coleman immediately announced that funds to build the Dulles Access Road extension would be requested from Congress. The cost of the extension was estimated at $35 million several years ago, but bids came in low.