Construction crews have completed work on an $8.6 million project to eliminate the intersection of Arlington Boulevard (Rte. 50) and Gallows Road, which since construction began two years ago has been listed as the most dangerous crossroads in Fairfax County.

Beginning Monday, Virginia transportation officials said, motorists on westbound Rte. 50 will be able to drive under Gallows Road as part of the state's long-awaited plan to separate the two heavily traveled highways. The eastbound lanes of the interchange have been open for two weeks. The state estimates that 69,000 vehicles pass through the intersection each day.

There will be no traffic lights at the interchange, described by state officials as the second of its kind in the state, allowing an uninterrupted flow of traffic through it.

Fairfax Supervisor Thomas M. Davis III (R-Mason), whose district lies a short distance from the interchange, said the highway redesign was welcome, but "long overdue." Davis added, "There's no question that the interchange will be a major factor in bringing down the number of traffic accidents and making traffic flow more smoothly in that area."

According to the Fairfax County Police Department, the intersection was the scene of 66 accidents that left 29 people injured in 1985. The year before, police recorded 48 crashes and 17 injuries there.

While the intersection was always a danger to motorists, it was the reconstruction project that turned it into the most perilous. Gallows Road traffic was rerouted to a temporary roadway parallel to it, creating massive traffic jams and leading to numerous accidents, police said.

David Gehr, a district engineer with the Virginia Department of Highways and Transportation, acknowledged the problems in a statement but said, "I think motorists will agree that it was worth the inconvenience."

In addition to eliminating the intersection, engineers widened Rte. 50 from two to three lanes in each direction from west of Williams Drive to east of the Capital Beltway. Gallows Road was widened from two to three lanes in each direction from Gate House Road to O'Connell Drive.

Two left-turn lanes and one right-turn lane also were built on Rte. 50 to carry cars onto ramps leading to Gallows Road. The project, completed with state funds appropriated under a six-year road improvement plan, has been on the drawing boards since the mid-1970s.