The last stretch of I-66 may have opened to vehicular traffic today, but bicyclists have been enjoying its benefits since summer, thanks to a $2.5-million "interstate" bicycle trail unlike any in the country.

The new trail, which parallels I-66 for 8.5 miles between Falls Church and Rosslyn, features a collection of overpasses and underpasses, banked curves and cloverleafs that has already won it an award as "perhaps the best new bikeway in the country."

In addition, Arlington and Virginia officials were given awards for the bike trail last month by the Washington Area Bicyclist Association (WABA) which, with 1,000 members, is the nation's largest bike commuter group.

The bike path even has its own miniature street lights, tiny street signs and "interchanges" with almost all county streets that abut the new four-lane highway.

Bike enthusiasts see it as a crucial link in the region's growing network of bike paths. Since its opening this summer, it has become popular with large numbers of Arlington residents and bike commuters--including many who unsuccessfully fought the construction of I-66.

The Virginia Department of Highways and Transportation promised to build the bike trail as one of the concessions for getting approval and funds for I-66 from the federal government, which had vetoed the state's original plans.

"I was against I-66 and I'm still not crazy about it, but this trail is okay, even if it's a little hilly and too many people walk their dogs on it--if you know what I mean," said a State Department official who daily commutes 12 miles round trip on his bike.

The "Custis Trail," as the bike path is called, is the counterpart of the Martha Custis Parkway, as Virginia highway officials have dubbed the controversial section of I-66 inside the Beltway.

The trail provides something of a downhill roller coaster ride into Washington, and a noticeably hillier return, with sound barriers and stone walls on one side and landscaped grassy hillsides on the other. Virginia is spending close to $1 million a mile to landscape the highway. Most of the work will be done next spring.

In some places there is a paved bike trail on both sides of I-66. Next year there will be a third parallel paved trail as well, at least for a short distance, when the Northern Virginia Regional Park Authority paves the adjacent Four Mile Run trail in Arlington. The present gravel trail is the eastern end of the park agency's 44-mile-long bike trail that roughly follows an old railway line between Alexandria and Purcellville in western Loudoun County.

The I-66 trail is expected to be used more than the paved Four Mile Run trail, however, because it provides a direct route into Washington.

The I-66 path puts cyclists onto Rosslyn sidewalks near Key Bridge, where they can cross into Georgetown. But a new $700,000 bike-pedestrian bridge to Roosevelt Island from Rosslyn is expected to be constructed within the next year, providing Arlington's first pedestrian access to the island in 50 years and also creating a pedestrian and bike access to Theodore Roosevelt (I-66) Bridge.

Not all news of the new I-66 bike trail is good, however. Arlington police reported a woman was abducted at knife point on the trail Oct. 26 and raped in a vacant town house project under construction near the trail. She was walking home in the dark from the Ballston Metro station along one of the few unlit sections of I-66, police said. The town house developer has since installed lights in the project.

Word of the rape has alerted families living near the trail, but not deterred use. Betsy O'Connor, who lives on nearby Jacksonville Street and pushes a baby carriage on the path, said she had heard of the rape but still takes walks of "at least a mile a day because it's good exercise" and because the trail is the nicest place in the neighborhood to walk. Curtis U. MacDonald also walks on the new trail with his wife "almost every day, because it's an all-weather trail and a good place for us senior citizens to take a daily constitutional," MacDonald said.

"We opposed I-66, and it has turned our dead-end street, Patrick Henry Drive, into a major crossover street. The highway may help traffic on local streets but not on crossover streets," he said. "But the trail--that's a major improvement."