The computerized traffic control system under construction in Northern Virginia on Shirley Highway and I-66 probably won't be finished by the Dec. 27 deadline, even though the state is fining the contractor $600 a day for the delays, Virginia Highways and Transportation Department officials said yesterday.

The $21-million project, begun in February 1982, is designed to maintain an even flow of commuter traffic by placing traffic lights on the highways' entrance ramps. Television cameras also are being installed along the routes to monitor traffic, particularly during rush hours and accidents.

Don Griffin, project manager for Bryant Electric Co., the Arlington-based contractor for the system, said yesterday that he believes the company will meet the December deadline, an extension of almost a year from the original completion date.

"I certainly don't feel those fines -- $18,000 a month -- are justified," said Griffin. "Many of the delays were the highway department's fault."

Griffin said the department piled on extra work after the contract was drawn, demanding more equipment testing, signs and television cameras.

"This equipment is the most sophisticated of its kind in the country today," Griffin said. "We are diligently testing it, but it's time-consuming."

"A contract is a contract," said E.E. Hull, assistant district engineer for the Virginia Highway Department. "They missed two milestones and for each of those they should pay $300 a day." On July 1, Bryant failed to meet the deadline for partial installation of the computer system and on Oct. 1, an equipment testing deadline was missed, Hull said.

The elaborate traffic-management system includes sensors buried in the ramp pavement, which trigger a central computer to respond to traffic patterns. By controlling the traffic lights, the computer can restrict the number of cars entering the highways.

The city of Alexandria and other Northern Virginia jurisdictions believe that this plan will cause backups on the ramps and divert highway traffic onto their streets.

"In theory, it's supposed to regulate the flow of traffic," Alexandria City Manager Doug Harmon said. "What it will do is benefit the jurisdictions further out and hurt those closer to the District."

Despite opponents, the project has the full support of the Federal Highway Administration, which is financing 90 percent of its cost, said Don West, an FHA highway engineer.