The Fairfax County Board of Supervisors joined Alexandria officials yesterday in questioning the effectiveness of a $22.9 million traffic control system Virginia highway officials plan to implement later this year on Shirley Highway and I-66.

The board adopted without dissent a resolution, proposed by Board Vice Chairman Martha V. Pennino, that "strongly questions" the traffic signals being installed on ramps leading to Shirley Highway. The board said it is worried about delays the controls may create for Fairfax commuters when they become operational this summer or fall.

The Fairfax board's complaints followed a similar protest by Alexandria officials. In December, the Alexandria City Council unanimously approved a resolution opposing use of the traffic-control devices during rush hours.

The Virginia Department of Highways and Transportation is installing the ramp meters as part of a computerized $22.9 million system aimed at improving the flow of traffic on Shirley Highway (I-395) and on the recently opened 10-mile section of Interstate Rte. 66 from the Capital Beltway to the Theodore Roosevelt Bridge.

The system was previously scheduled to start operating in the spring, but highway officials said yesterday it has been delayed by unexpected difficulties in installing communications cables. More than 90 percent of the system's cost has been financed from federal transportation funds, officials say. The remaining cost has been paid for by the state.

The recent controversy over the long-planned system stems from fears by some Northern Virginia officials and commuters that the signal lights may delay drivers at ramps leading to the two highways and may cause traffic backups in surrounding neighborhoods.

Highway officials contend that any delays at the ramps will be minor, lasting no more than 6 or 7 minutes at the most congested entrances. Moreover, they say, any slowup at the ramps will be more than offset by swifter traffic flow on the highways.

Fairfax board members said yesterday that the system unfairly favors long-distance drivers over county commuters, who may get stuck at ramps. "It gives the advantage to the interstate trucker," said Supervisor Audrey Moore of Annandale.

County officials also complained that Fairfax had no role in planning the new system. "We would have liked to have a little more direct involvement in it," said Supervisor Joseph A. Alexander, who represents the Lee District south of Alexandria.