A new $8 million computerized traffic signal system in Alexandria and Arlington is expected to be completed late this spring, but until then there may be some traffic delays for commuters driving through the two jurisdictions.

When the interconnecting grid of signals is complete, motorists could save as much as 10 to 20 minutes when driving along Alexandria's and Arlington's main arterial roads, according to James B. Stone, an engineer for the Virginia Department of Highways and Transportation, which is supervising installation of the new system.

However, the "shortcuts" many motorists now take through residential neighborhoods should take longer, because traffic signals will be timed to speed traffic along main routes and slow it along side streets.

Alexandria traffic in recent weeks has been "about as bad as it can possibly be," said William Long, assistant chief of the city's transportation division, because the old underground cables that connected and synchronized the city's traffic lights have been torn up.

"While new underground cables are being installed, each signal is on its own timed sequence," Long said. The problem is that the lights' individual timers frequently get out of synch, causing traffic back-ups.

The traffic-signal systems are similar to those installed recently in Richmond, Norfolk and several other Virginia cities but are not as complicated as Washington's soon-to-be modernized "grid" system, which controls virtually all 1,200 signalized intersections in the capital.

The District has had a centrally controlled light system for almost 30 years. In the early 1970s it installed one of the nation's more sophisticated computerized signal systems, but the new system "was scrapped about five years ago when the telephone company raised its rates about 600 per cent for use of its underground cables," according to Gary Wendt, chief of Washington's traffic operations division. A consultant is now studying improvements to the original city system, which is operated by old underground cables and radio-controlled lights.

The new Alexandria and Arlington systems are primarily "straight-line" systems along major corridors, with few "grid" systems of criss-crossing signalized streets. However, they will have many sophisticated aspects, including sensors built into the pavement to tell the computer how fast traffic is moving and where there are problems. Federal and state grants are paying for 95 per cent of the cost of the Alexandria and Arlington systems.

James B. Stone, state traffic engineer in charge of the Alexandria project, said a major advantage of the two Northern Virginia systems is that they will use a standardized federally designed computer model now being used across the country, which will be easy to maintain.

In Alexandria, city-owned underground cables will connect signals with a computer room now being built in the City Hall courtyard. The computer will control most of the city's 234 signalized intersections, including all lights east of the railroad tracks and Metro subway line, and along the city's two main north-south corridors, Route 1 and Washington Street. It will also control lights along Duke and Van Dorn Streets. Also, in several years it will control a mini-computerized light system the city presently is installing on its own along King Street.

In Arlington, the county wil lease recently installed underground cables owned by a private cable television firm, Arlington Cable Television Corp. (ARTEC). Over the next 10 years the county will pay ARTEC $237,250 for use of its cables.

All of the county's 200 intersections with traffic signals will be controlled under the new system, but most of these signals are along major routes such as Route 50 and Route 1, Columbia Pike, Lee Highway and Wilson and Washington Boulevards. The county has small grid systems in Rosslyn, Clarendon and Crystal City. Lights at all I-66 intersections in the county also will be coordinated under the new system.