John Van Vliet commutes from Harpers Ferry, W.Va., to Alexandria every day. And almost every day he stops at Dulles International Airport, not to catch a flight, but to buy a 99 cent cup of coffee at the sole gas station on airport property. He makes sure to get a receipt.

That receipt, according to Van Vliet, allows him to drive on the nearly traffic-free Dulles Access Road, avoiding the tolls and traffic of the parallel Dulles Toll Road. It shaves about 50 minutes off his morning commute.

"It's legal if you do business with the airport," Van Vliet said with a smile. "I have my receipt with me."

Van Vliet is taking advantage of a loophole that allows drivers on airport business to use the access road, which was designed to funnel airport passengers to the airport from the Capital Beltway. Since there is no official definition of "airport business," Van Vliet and hundreds of others drive their vehicles through the giant loophole every day.

In the traffic-clogged Washington area, every commuter is looking for an edge in the daily battle to get to work and home. The tips are passed across the dinner table and over cubicle walls. It could be a secret shortcut, using inflatable dolls to get around high-occupancy vehicle restrictions or stretching the definition of airport business.

"It's like commuter science," said Danielle Delgado, who uses the Dulles shortcut about three times a week. It saves her about 40 minutes in her commute from Purcellville to Old Town Alexandria. "You're thinking on the fly when you're hitting the gas," she said while stopping off at the airport one recent morning for a cup of coffee, a snack and a trip on the access road.

Because drivers on airport business are also allowed to use Interstate 66 during HOV-only hours, the loophole allows single drivers an easy ride into the District or out to the airport 35 miles away. During rush hours, only vehicles with two or more passengers are allowed to use I-66 inside the Beltway.

Although drivers have been using the airport loophole for years, airport police say the increasing number of long-distance commuters, the explosion of residential growth beyond the airport and worsening traffic on the Dulles Toll Road, which opened in 1984, have made the detour more attractive.

Police say they have even found Reston residents cutting through the airport, meaning the time savings is such that it makes sense to travel west to the airport to ride east on the access road.

The problem with what police call "backtrackers," a generic term that includes all commuters who cut through the airport, is likely to get worse when the Dulles Toll Road raises its rates to help pay for the planned Metrorail extension to the airport.

State Transportation Secretary Pierce R. Homer said backtracking is just one result of too many cars on too few roads. It is also a testament to the craftiness of drivers.

"The Washington area commuter is by far the most ingenious," Homer said. "They use loopholes, inflatable dolls . . . they get into cars with complete strangers and go west to go east. They do what they have to do to survive."