Work begins tomorrow on an 18-month, $6.84 million project to complete the widening of the Virginia part of the Capital Beltway, which will cause delays near the American Legion Bridge.

Virginia's 23 miles of the Beltway are four lanes in each direction, but there are points along a one-mile stretch between Route 193 (Georgetown Pike) and the bridge where the fourth lane is used by drivers entering and exiting from the George Washington Memorial Parkway.

About 160,000 vehicles a day travel between Route 193 and the bridge, one of the highest counts on the 64-mile Beltway.

When the project is finished in October 1991, Virginia transportation officials said, there will be four through lanes in each direction to and from the bridge, which carries the Beltway over the Potomac River.

As crews prepare for construction, officials will close at least two southbound lanes between 9:30 a.m. and 3:30 p.m. starting tomorrow and continuing for at least 10 days.

The road will be narrowed and restriped and concrete barriers will be put up on the southbound shoulder between the bridge and Route 193.

When all work is done, traffic heading south on the Beltway's Outer Loop from the bridge will have two options.

Through traffic can stay in one of the four through lanes, while local traffic wishing to exit at either the parkway or Route 193 will be diverted onto a separate roadway.

That separate roadway divides local traffic traveling between the bridge and Route 193 from through Beltway traffic with a concrete barrier.

Northbound traffic from Route 193 will have a new lane taking drivers directly to the George Washington Parkway.

At present, vehicles headed toward the parkway must merge onto one of the Beltway's Inner Loop lanes briefly before exiting.

After completion, there will be one northbound lane leading to the parkway and four through lanes for Beltway traffic.

In addition, officials said, another southbound lane will be added to the on-ramp at Route 193.