Fairfax County officials signaled their intention yesterday to go to court to block a costly traffic control system Virginia officials have planned for Shirley Highway and I-66.

The county's promise to fight the $22.9 million computerized traffic system came on the same day that highway officials said they will ease the Shirley Highway's carpool restrictions on an experimental basis. Under that proposal three-member carpools would be allowed to use the highway's express lanes during certain periods of the morning rush hour. Cars using those lanes now must have at least four riders.

While that decision was greeted with delight by Fairfax and Alexandria officials, the Fairfax board of supervisors directed their attorney yesterday to work with Alexandria in developing a lawsuit against the state's plan to install traffic signals on ramps leading to the two highways this summer. The Alexandria City Council, which is also troubled by the ramp proposal, last week raised the possibility of going to court over the issue.

Fairfax and Alexandria officials fear that the ramp metering will favor commuters from the outer suburbs and cause traffic to back up as motorists sit at red lights waiting to get on Shirley Highway (I-395) and I-66 inside the Capital Beltway.

"As a policy-maker, I think this thing is going to be a disaster," said Supervisor Thomas M. Davis, a Republican of Mason District.

Virginia highway officials have defended the metering project, saying it works well on California freeways and won't disrupt traffic on Shirley Highway, the region's busiest commuter artery.

Al Coates, a spokesman for the Virginia Highway and Transportation Department, said that details of the express lanes experiment have not been worked out, but that an approach proposed by state Sen. Richard L. Saslaw (D-Fairfax) during a meeting Friday with Gov. Charles S. Robb and Highway Commissioner Harold King is reasonable.

Saslaw suggested allowing three-passenger instead of four-passenger cars on the express lanes between 6 a.m. and 7 a.m. and 8:30 a.m. and 9:30 a.m., the portions of the morning rush hour when the express lanes are not fully utilized.

Coates and Saslaw said the experiment is dependent on the state's computerized system, which will include television surveillance, to signal when the express lanes can handle more cars, and electronic signs, to alert drivers when three-passenger cars are allowed. Coates said the experiment could start as soon as late June.

"If they try it and it doesn't work, it will put the argument to rest," said Saslaw. "But to simply say, 'It won't work,' without any type of trial is to stick your head in the ground."

Sen. Wiley F. Mitchell Jr. (R-Alexandria) said he wishes the experiment well and believes it probably will be a success, but "I am a little concerned that the decision is being made by everyone except the people using the lanes."

Del. Warren G. Stambaugh (D-Arlington) said he is afraid Fairfax politicians will use the experiment to urge that the express lanes be abolished. "I think a little test like this one is not, in itself, harmful," he said. But he said the test will lead to "three-passenger cars the entire time, and the next thing you know, backed-up cars belching exhaust into the lungs of Arlington County residents."