The good news is that, sometime next fall, the 3.7-mile Dulles Airport Access Road extension will open, linking at long last the 13-mile-long Dulles Airport Access Road with I-66. That's good news because it will give airline passengers and a few commuter buses a direct, rapid route between downtown Washington and Dulles Airport.

The bad news is that the access road may well prove a bonanza for some 2,500 to 3,500 rush-hour commuters who are known as the Dulles Access Road Outlaws.

And that may mean more traffic on I-66 at rush hour than there should be.

The problem stems from the fact that, under regulations restricting rush-hour use of I-66 to car pools, an exception has been made for anyone bound for Dulles Airport. While everyone else on I-66 at rush hour must be riding in a vehicle carrying a minimum of four passengers, vehicles bound for Dulles will not be required to do so.

The question authorities are asking themselves is: How will anyone be able to tell the legitimate Dulles traffic from the "outlaws," who use the restricted access road as a private driveway to and from the nation's capital?

The anonymous "outlaws", all of whom are commuters from western Fairfax and beyond, got their nickname because they sneak onto the access road masquerading as people bound for Dulles Airport, which is restricted to airport traffic only. They drive west to the end of the road at the airport, circle around, then zip all the way to the Capital Beltway and Dolley Madison Highway, neatly bypassing the heavily traveled Rte. 7 and congested Tysons Corner area.

Highway officials say it will be impossible for the 23 state troopers enforcing rush-hour car pool rules on I-66 to distinguish between legitimate airport traffic and commuter scofflaws, so the wily access road commuter, alone in his auto, will be able to merge into I-66 and sail downtown in solitary bliss.

Unless, of course, the Federal Aviation Administration, which owns and operates the road, figures out a way to isolate the commuters. That won't be easy, although FAA spokesman Dave Hess said the agency will have a solution in hand well before the road extension opens.

"Backtracking," as the practice is called, has plagued the Federal Aviation Administration for years, and construction of the access extension only makes the practice more attractive. Despite periodic efforts to halt or at least slow the flow, "We don't have much of a fix on it yet," said Hess.

"It's hard," agreed an FAA engineer. "We don't have a method by which we can count them, much less catch them."

The reason the backtracking problem has been difficult to solve, according to FAA officials, is that there are at least three different ways commuters can enter the airport to execute the crucial U-turn. An indication of the "outlaws' " impact is that traffic volume at the usually deserted airport jumps by 60 to 70 percent during rush hour, according to airport officials.

A study by FAA planners has outlined several enforcement options, but decisions take time, Hess cautioned. FAA engineers say that, of the several options being considered, one of the ones most likely to succeed is simple license plate surveillance.

"If a certain license number keeps popping up at certain periods of the day," explained FAA engineer Hank Mahns, "we would notify the troopers."

That could solve the problem, and certainly the highway department is counting on it. "We think they'll come up with something," said Virginia highway department official David Gehr. "They have to."