The federal Department of Transportation has endorsed a controversial $4.9 million computerized traffic light system to regulate the flow of automobiles onto Shirley Highway, a project that opponents say will back up traffic and congest neighborhoods in Alexandria and Fairfax.

Federal support for the system, which is scheduled to be in operation this fall, was disclosed in a letter to Rep. Stan Parris (R-Va.) from R.D. Morgan, executive director of the Federal Highway Administration. Morgan said in the letter that he concluded that the traffic signals on the north and south ramps would not significantly impact upon traffic, air or noise in the Alexandria and Fairfax suburbs.

The decision was a disappointment to local officials who originally viewed Transportation Secretary Elizabeth Dole's announcement last month to review the system as a signal that the federal government might block the project. In addition to traffic backups and congestion, critics say that the signal system will give preference to commuters from outer suburbs who have entered Shirley Highway at points at which traffic signals have not been installed.

The $4.9 million ramp-metering project for Shirley Highway, or I-395, will involve installing traffic lights on 11 northbound and eight southbound ramps on the highway. It is part of a $22 million traffic management system for I-395 and I-66 that evolved from a 1976 study of traffic congestion. The system also includes traffic surveillance and widening ramps.

Supervisor Thomas M. Davis, Republican of Mason District, where neighborhood groups oppose the lights, said that the letter from the federal highway department "doesn't resolve the issue as far as we're concerned. They federal transportation officials are not the final word. The court is."

Davis said he would recommend that the Fairfax Board of Supervisors proceed with its threat to sue Virginia to prevent the system from being activated this fall as scheduled.

Asked about the possibility of a lawsuit, Fairfax Board Chairman John F. Herrity, a Republican who has been criticial of the system, said: "We have to go back and talk to our attorneys."

Herrity said that he would favor a suit if Fairfax had "reasonable prospects" of winning, adding: "The last thing we want to do is file a lawsuit that doesn't have possibilities."

Although the federal government refused to intervene to stop the project, it agreed to work with state transportation officials who will be monitoring traffic and to "fine-tune" the system to "minimize any disruption to local traffic."

A spokesman for Parris said that the Northern Virginia congressman remained "somewhat skeptical as to the workability of the ramp-metering system, but was pleased at Morgan's assurances that the system will be carefully monitored."

In his letter, Morgan said that an earlier analysis indicated that, since there were few alternative routes that users of Shirley Highway could opt for if they were delayed by red lights at the ramps, there would be a minimal amount of neighborhood traffic problems. He also said that the traffic lights at the ramps would be set or even turned off to keep traffic flowing smoothly.

He said that "if the system does not permit ramp congestion to spill over onto adjacent streets, its impact would be minimal."

"Pure bureaucratic gobblygook," retorted Davis. "Clearly it is going to have an impact on traffic."