Traffic in Alexandria, already bumper to bumper in many places during rush hours, could come to a screeching "grid-lock" halt within this decade, city officials predicted yesterday, unless drastic measures are taken to restrict development.

The city claims it suffered a traffic setback this fall when a developer won a court battle to construct an office complex with parking spaces for 450 cars beside the already traffic-clogged George Washington Memorial Parkway at Slater's Lane.

"But that is only the tip of the iceberg," City Council member Donald Casey said yesterday.

Current development along Alexandria's north waterfront and around its new Metro subway stations will soon bring traffic at five downtown intersections to what transportation planners call Level F, according to new traffic projections. At Level F, traffic exceeds the capacity of a road and barely moves.

City planners are now focusing on another major development, proposed for a tract on the grounds of the huge Potomac Yard of the Richmond, Fredericksburg & Potomac Railroad Co., one of the nation's largest freight train assembly facilities.

Potomac Center, as the proposed development is called, would be built along the George Washington Parkway just south of National Airport. Current zoning on the land permits a complex of office buildings with five or six 1,000-car parking garages.

"If Potomac Center is built . . . under present zoning . . . traffic will come to a virtual standstill," said Dayton Cook, the city's top transportation planner.

Potomac Center would have direct access to the parkway with a major new intersection -- agreed to by the National Park Service, which owns the highway -- in a land swap for part of Dyke Marsh south of Alexandria.

Cars from Potomac Center would triple present rush-hour traffic on the parkway, which already reaches Level F during morning rush hours, when it carries more than 3,600 cars an hour.

Cook said yesterday the RF&P "also has plans to develop the yard along Rte. 1, where they could build a project even bigger than Potomac Center, with even more cars.

"It could be as dense as Crystal City the massive development along Rte. 1 to the north , but a kind of sawed-off Crystal City since our 77-foot building height is half the 150-foot height in Arlington," Cook said.

RF&P Potomac Yard Superintendent John F. McGinley confirmed yesterday that the railroad is considering development along Rte. 1, but said, "It is very remote. We're looking a long way off, probably not till the year 2000 . . . we'll develop Potomac Center first."

The city is now searching for ways to protect itself from the side effects, especially traffic jams, created by new developments.

The City Council already has introduced legislation to require developers of every new project that will have more than 100,000 square feet of space to submit a traffic-impact study.

An amendment to that measure was added last week to require that developers obtain special-use permits from the council for any project that would reduce traffic to Level F. Public hearings are expected to be held in January.

The Slater's Lane office complex with its 450-car garage demonstrated the city's present helplessness. Under current city and state laws, the Circuit Court ruled, Alexandria must routinely approve any development that meets city regulations. It may not consider the traffic impact a new development might cause because city zoning laws don't mention traffic or impact on traffic.

The city has appealed the court decision, but the Virginia Supreme Court will not decide whether to hear the case until next summer. Even if it does hear it, it will not make a ruling until 1984 at the earliest, City Attorney Cyril D. Calley said yesterday.

This leaves both the city and the developer, Potomac Investment Associates, in difficult positions, neither knowing -- at least until next summer and possibly not for three years -- whether the City Council can consider traffic impact when considering developments.

The city plans to ask the state legislature to permit Alexandria to create special "urban transportation districts" in which it could require developers to help solve transportation problems they create and force them to help pay for such things as feeder buses to Metro.

The city also may change its parking requirements. "Instead of regulations that require Potomac Investments to build a 450-car garage, we could limit them to no more than 200 cars," Cook said. That might force tenants to make greater use of Metro or dictate construction of a smaller development.

The city already has lowered parking requirements near the King Street Metro station, in exchange for developers' promises to foster car pooling.