Washington will move about 20 minutes closer to some Northern Virginia suburbs Wednesday morning.

That's when the final 10-mile section of Interstate Rte. 66 from the Capital Beltway to the Theodore Roosevelt Bridge is scheduled to open after more than 20 years of bitter controversy, court challenges and shifting government policies.

The four-lane highway, which will be restricted to car pools and buses during rush hours, remains a focus of debate and uncertainty. Critics who fought the road during the 1970s complain that it will lead to increased traffic on nearby streets, unwanted commercial development, noise, dirty air and other problems in residential Arlington communities.

Advocates praise the I-66 extension as the centerpiece of a balanced transportation corridor, interconnected with Metrorail and bus lines and adjoined by hiking and biking trails. Much of the support for the road came from Fairfax, Prince William and other outer suburbs where commuters long have sought more direct access to Washington.

Because of the car-pool requirement, officials say it is difficult to predict how many commuters will use the road, officially named the Custis Memorial Parkway by Virginia highway officials. Estimates are that it will carry up to 48,800 vehicles a day by 1984, a relatively modest amount of traffic compared with other major commuter arteries in the suburbs.

From 6:30 a.m. to 9 a.m. weekdays, eastbound traffic will be limited to buses and four-member car pools. The same restrictions will apply to westbound traffic between 3:30 p.m. and 6:30 p.m. Heavy-duty trucks--those with two axles and six or more tires--will be barred from the road at all times. Virginia State Police say they will patrol heavily the ramps to the highway and violators will face $50 fines for failing to comply with the restrictions.

Highway officials say traffic is likely to be moderate this week, largely because of a seasonal drop in commuters as the Christmas holidays approach. They foresee no rush-hour bottlenecks on I-66 in coming months aside from those that already exist, chiefly where I-66 meets the Capital Beltway and at the Theodore Roosevelt Bridge. Even at the busiest commuting hours, traffic is predicted to move smoothly along the highway for the next two years.

Those official forecasts have stirred dispute, even among such highway advocates as the American Automobile Association. "Within a year, you're going to start seeing traffic jams on I-66," said Tom Crosby, spokesman for the AAA's Washington regional office. "It's going to be heavier than anyone anticipated." Crosby cited a trend toward increased car-pooling and population growth in outlying suburban areas.

Officials say the I-66 extension will reduce commuting time by about 20 minutes for many rush-hour drivers and will help lessen rush-hour traffic on several congested Northern Virginia arteries, including Arlington Boulevard (Rte. 50), Lee Highway (Rte. 29-211), and the George Washington Memorial Parkway.

Next fall, when a connecting road to the Dulles Access Road is completed, I-66 will provide a direct route between downtown Washington and Dulles International Airport. Commuters heading to or from Dulles will not be required to comply with the car pool requirements once the so-called Dulles connector is opened, officials say.

One unanswered question about I-66 is whether all Washington-bound commuters on the George Washington Memorial Parkway will continue to be allowed to use a ramp to the Roosevelt Bridge. Virginia highway officials have sought to restrict the ramp to four-member car pools during the morning rush hours, saying the ramp actually leads not to the bridge but to a restricted portion of the highway. The National Park Service, which owns part of the ramp and runs the parkway, has refused to impose the restriction. Virginia officials say they hope to have the issue resolved before Wednesday's opening.

The AAA has urged drivers to study the new route before venturing onto it at rush hour. "It's a difficult road to use at first," Crosby said. "It's going to alter traffic patterns in all of the neighborhoods where there are exits."

The Arlington County Board, which long opposed the highway, now appears divided over the road. Several board members recently expressed concern about how the highway will affect neighborhood traffic patterns and whether lights along I-66 will glare into adjoining communities. But County Board Chairman Stephen H. Detwiler says: "I'm hoping that it's going to relieve some of the commuter traffic which is now going through our residential neighborhoods."

In the District of Columbia, where officials previously objected to the highway, transportation administrators now say the road's opening probably will have no significant impact. The Theodore Roosevelt Bridge, they say, is already so jammed during rush hours that it cannot funnel any more cars onto city streets. "It's like trying to make 100 gallons of water go through a straw," said D.C. Transportation Director Thomas Downs. "You can't make any more traffic go across that bridge."

In the 26 years since I-66 was first proposed by the Virginia Highway Commission as an eight-lane superhighway, dramatic changes have occurred. Costs for building the final stretch inside the beltway skyrocketed from an estimated $25 million in the 1950s to about $275 million today.

A decade of environmental activism and controversy over the road led to a compromise between Virginia and federal officials that restricts the road's use and limits the highway to four lanes. Noise barriers have been constructed in subdued earthen hues to soften the impact on adjacent communities. Trees, vines and shrubbery have been planted beside the roadway. Fences, drainage areas, shoulders, light posts, bridges and other structures have been designed with the esthetic aim of blending the highway into its surroundings.

The Washington area's Metrorail network, planned and partly built during the same decades as I-66, has also left its mark on the highway. Orange Line tracks will run in the I-66 median, where a roadbed has been laid and stations are now being constructed. Since June, Metrobuses have traveled on part of the new I-66 extension, reducing commuting time to Washington from beyond the Beltway by 5 to 20 minutes, according to transit officials.

Next spring the new highway will be regulated by a computerized traffic-control system with special television-monitoring equipment, a companion to a similar system being installed on Shirley Highway (I-395). When the system goes into operation, ramps to I-66 will be regulated by signal lights, designed to maintain an even flow of traffic.

"It's been a good learning process for the highway engineers in Virginia," said David R. Gehr, assistant Northern Virginia administrator of the state Department of Highways and Transportation, as he sped along the nearly completed parkway on a recent afternoon. "There are ways to fit a freeway into the urban environment without damaging the environment. It may cost a little more, but to the people in the area, it's worth it."

In the highway's tangled history, several key decisions were made by former U.S. Secretary of Transportation William T. Coleman Jr. after a court challenge by the road's opponents. Coleman initially barred the I-66 extension in 1975, terming it obsolete in view of Metro's expansion and other shifts. He later reconsidered and approved the scaled-down version in 1977 with the car pool and other restrictions.

A major factor in Coleman's change was an agreement by Virginia officials to provide millions of dollars for construction of the Metro system. Highway workers have built an estimated $75 million worth of Metro facilities, mainly in the I-66 median. In addition, Virginia turned over $51.7 million to help finance other Metro projects. The money came from the state's allotment of federal highway funds.

In recent years, battle lines over the highway have shifted only slightly.

John F. Herrity, chairman of the Fairfax County Board of Supervisors, praised the I-66 extension a few days ago as a benefit for commuters, a step toward quicker access to Dulles International Airport and a spur to economic development, especially in the Dulles, Reston and Tysons Corner areas. "It's a significant factor in the future economic health of Fairfax County," said Herrity, who got the Fairfax board to drop its opposition to the highway after he was elected chairman in 1976.

Emilia Govan, a founder of the Arlington Coalition on Transportation whose lawsuit blocked the road in the early 1970s, still objects to the highway. She described the court fight, however, as worthwhile. "Citizen involvement at least ameliorated some of the intrusive effects that the highway would have had," she said.

Dismay persists in some Arlington neighborhoods. "I fought the road for years, and when it opens, it's going to be stinky and noisy," said Ned C. Helf, who lives about 250 feet from the highway and is president of the Maywood Community Association.

"You can get used to an elephant living in your back yard," complained Benjamin L. Bullock, an activist in the Waycroft-Woodlawn community. "But basically your life is never the same again."

Some critics contend that if traffic on the I-66 extension is light, political pressure will mount to relax the car-pool requirement. If traffic is heavy, they argue, Arlington commuters may be kept off the road by the computerized ramp lights. "The people who will be left out are the people near the river, which is us," said Marianne Karydes, an opponent of the road.