The opening of the $285 million Interstate 66 extension between the Capital Beltway and the Theodore Roosevelt Bridge has been tentatively scheduled for Dec. 22, Virginia highway officials announced yesterday.

Donald Keith, regional director of the state Department of Highways and Transportation, said the December opening is based on a tight construction schedule that could be pushed back if the weather doesn't cooperate.

The extension, officially known as the Custis Memorial Parkway, was first approved for construction in 1977. Its estimated cost of $285 million is about $115 million above original projections.

Metrobuses have been using the section from Fairfax Drive to Sycamore Street in Arlington since November. In June, buses will use I-66 from the Capital Beltway on their way to the Ballston Metro station.

Keith said traffic inbound to Washington from 6:30 to 9 a.m. will be limited to buses, emergency vehicles and four-person carpools. The restrictions will apply to outbound traffic between 3:30 and 6:30 p.m. At no time will the roadway be open to heavy trucks.

Keith said the restrictions were imposed by former Secretary of Transportation William T. Coleman as a condition for releasing federal construction funds.