The first permanent computerized traffic light system in the Washington area -- 130 intersections in Alexandria's Old Town and along the Duke Street corridor -- has begun operating after years of disruptive street work, and engineers say that cars are starting to move more smoothly in parts of the city.

An official verdict on whether the controversial $4 million investment really works will have to wait until a vehicle count begins in September. "We're encouraged. But it's still a rough row to hoe," says city transportation official Charles Kenyon.

Alexandria's use of the new technology comes as other governments in the area are proceeding with similar plans. Arlington County is tying 200 intersections together, while D.C., which experimented with the concept in the 1970s, hopes to computerize all of its 1,250 controlled intersections within five years.

Alexandria traffic engineers, combining telephoned complaints from motorists and their own observations, now are trying to chase bugs out of their computer, which is located in an air-conditioned building erected for it in a City Hall courtyard.

The hardware works, technicians say. The challenge has been to correct shortcomings in the programming, which attempts to accomplish the enormously complex task of making lights at the 130 intersections act in harmony and create progressions of green.

This shakedown has been "more difficult and time-consuming here than what anybody anticipated," says city manager Douglas Harman, who acknowledges that the system remains controversial because of red-light wrinkles that frustrate some drivers.

For instance, westbound traffic at one Cameron Street crossing was getting only five seconds of green late at night until recently. "We never found out until we had a council meeting that ran late one night," Kenyon said, and city officials had to drive through it.

Harman and Kenyon say progress is being made. Harman said he recently was "astonished to drive from the King Street Metro station to City Hall without having to stop at one light . . . . I almost fainted." Cameron Street, Prince Street and the intersection of Duke and Walker streets are cited as other places where cars are moving with fewer delays.

Even at their best, however, computers are no cure-all for traffic jams, engineers point out. Driver psychology and limits of space make it impossible for everyone to get green.

About 200 intersections in Alexandria have signals. Before construction of the new system began in 1979, only 30 (mostly on Rte. 1 and Washington Street) were linked and synchronized, using bulky mechanical equipment dating from the 1950s.

This spring, after 2 1/2 years of construction chaos, the 130 intersections chosen for the system finally were linked to the computer. Now engineers are puzzling over how to balance north-south arteries, used heavily by Fairfax County commuters, with east-west streets, which locals use more often. Some citizens complain that east-west flow is being penalized.

Currently, the computer has five timing sequences, one each for traffic levels expected in the morning rush, the evening rush, lunch time, off-peak hours and late at night. Operators can tell whether any given light is red or green by glancing at a wall-size electric map.

The computer now shifts patterns at preset times. This summer, it will be "traffic responsive." Fifty-three buried detectors will count passing cars and report to the computer, which will change sequences automatically based on actual traffic levels.

The computer can handle up to 30 timings for special needs such as weekend traffic and events such as parades. On a recent Thursday evening, when hundreds of people, many of them with cars, converged on City Hall for the inauguration of the City Council, programmers for the first time manipulated timings for a special event.

The lights' evening rush-hour timing, which normally ends at 8 p.m., was extended until midnight.

Engineers have long debated whether responsive systems work any better than those that follow a set program. Kenyon says Alexandria might have chosen something simpler but selected a responsive one to get federal aid. As result, the city is paying only 10 percent of the cost, with state and federal aid covering the rest.

The new equipment and two new employes to run it will add about $80,000 to the city's operating budget, eating into any savings in time and fuel that result from it.