A plan to open Dulles Access Road to carpools would slightly reduce rush-hour traffic congestion, gasoline consumption and air pollution in auto-clogged Northern Virginia, according to a study released last week by the Federal Aviation Administration.

The 51-page environmental assessment also noted that the plan would save Reston-area communters 30 minutes a day in driving time to and from Washington.

But opening the 13-mile road to carpools could have some disadvantages, the study says. While decreasing congestion on commuter roads such as Rte. 7, increased use of the access road may cause delays for airport traffic. r

If the access road -- now theoretically available only to airport traffic -- is open to carpools during rush hour, it will attract 600 to 1,000 carpools a day, according to the FAA study.

The study also predicts that some commuters who now use mass transit such as Metro and Reston buses will change to carpools. When the Metro subway line to Vienna opens in 1984, it may attract "slightly" fewer passangers that predicted because of the availability of the access road.

The access road would be opened to carpools by the end of January, if the FAA Prosposal is approved by the National Capital Planning Commission at its meeting today, according to Frank J. Conlon. Conlon is chief of engineering for the FAA's Metropolitan Washington Airports, the division that operates Dullas and National Airports.

Carpool use of the access road is proposed for only five years, until the Dulles toll road is built beside the access road. Carpool use could be stopped before then if the toll road is completed sooner of if the FAA finds that "carpools create congestion or interfere with" airport traffic, the environmental study states.

When the number of carpools rises above 1,000 a day on the access road, it "could jeopardize the road's level of serivce to airport patrons," the report states. It predicts that carpools will then mean the difference between relatively "free-flowing" traffic on the access road and a "stable flow" of traffic with "slight delays."

The $60 million Dulles toll road, to be funded by state bonds, was authorized by the Virginia legistature earlier this year, but is not expected to be completed until 1984 or 1985. State highway officals already have begun an environmental review of the four-lane toll road.

The toll road will run from Dulles Airport to Rte. 123. Public hearings on the road, part of the environmental review process, are expected to be held in January 1981.

The Dulles Access Road corridor is expected to become a major communter route by 1982 -- even before the toll road opens -- when the controversial section of I-66 inside the Beltway is completed. The Dulles Access Road is being extended to connect with I-66, although the completion date has now been set back to late 1983 or early 1984.

Both I-66 and the 3.5-mile extension of the access road will be open only to carpools and buses during the rush hour. Plans have been dropped for entending the Metro subway line to Dulles, although the 3.5-mile extension will leave room for a subway line if that decision is reversed. The West Falls Church Metro station, due to open on the Orange Line in 1984, will be at I-66 and the Dulles Access Road extension.

The FAA estimates carpools using the access road will average 4.5 passengers per car, since a minimum of four passengers will be required to enter the road. The FAA insists the minimum must be strictly enforced by Virginia State Police or Fairfax County police.