Virginia's computerized $26 million traffic control system appears to have eased rush-hour congestion on Shirley Highway but not on Rte. I-66, according to data compiled by state highway officials.

"The average driver is saving time," said Thomas F. Farley, assistant Northern Virginia district engineer for the state Department of Highways and Transportation.

The electronic system, which began operating June 5 on Shirley Highway (I-395) and I-66 after a protracted controversy, has significantly reduced weekday delays on Shirley Highway, especially between 8:30 and 9:30 a.m., near the end of the rush hour period, the statistics indicate.

Congestion also appears to have decreased, although less markedly, during the previous hour, considered the height of the morning rush hour on the highway, the figures show.

The data provided no evidence of any similar reduction in rush-hour backups on I-66 inside the Capital Beltway. Farley said it was "too early to tell" whether the system will eventually lessen traffic congestion there.

At the same time, officials said, the system has allowed police and other emergency workers to respond more quickly to accidents, hazardous weather conditions and other incidents on the two highways.

The system, which includes signal lights on entrance ramps, was designed to even the flow of traffic on the two highways in an attempt to reduce congestion, prevent accidents and provide quicker trips for commuters.

Computer-regulated signal lights, closed-circuit television cameras and electronic traffic signs were installed on an 11.5-mile section of Shirley Highway between Springfield and the 14th Street bridges and on a 10-mile stretch of I-66 between the Beltway and the Theodore Roosevelt Bridge.

Critics, including Alexandria and Fairfax County officials who unsuccessfully challenged the system in federal court, initially warned that the new devices could lead to backups at ramps, congestion on nearby streets and delays for commuters in close-in suburban areas.

Today, the controversy appears to have faded. State, county, city and American Automobile Association officials say they have received relatively few complaints from commuters. State police and highway officials point to widespread compliance with the ramp controls by drivers.

"We haven't seen any unusual backups on any of our streets yet," said Alexandria transportation chief Charles E. Kenyon, who previously had expressed concern about the new system. Kenyon contended, however, that traffic problems may arise later.

Although an extensive study of the new system is not expected to be completed until spring, state officials said, some preliminary conclusions may be drawn from recent data. The statistics were compiled on six weekday mornings in August, September and October.

In broad terms, the figures were described as reflecting both the number of minutes during which traffic was relatively congested on any lane of the two highways and the number of miles over which backups extended. On three days, the system was running smoothly. On the other three, malfunctions had occurred.

Although major breakdowns have been relatively infrequent, officials said, the statistics compiled during the three mornings on which the system malfunctioned were viewed as indicating the level of congestion that would probably have occurred if the system had never been installed.

Congestion was found to have been markedly less extensive on Shirley Highway between 8:30 and 9:30 a.m. on the days when the system was functioning normally. "It proves that we've gotten the benefit," Farley said.

A less pronounced improvement was found on Shirley Highway between 7:30 and 8:30 a.m. Nevertheless, Farley said this trend appeared significant because planners had not expected any substantial decrease in congestion at the height of the rush hour. He described the results as a welcome surprise.

On I-66, Farley said, the data showed "almost no or minimal improvement." The system apparently has failed to lessen congestion on I-66 chiefly because the highway has relatively few entrances regulated by signal lights, officials said. Most cars enter that highway from outside the Beltway or from the Dulles Access Highway, where there is no signal.

Daily monitoring of traffic on the closed-circuit television system also has pointed to a decrease in congestion on Shirley Highway after 8:30 a.m., officials said. "We can tell the difference," said Jimmy Chu, a systems engineer who manages the traffic control center in Arlington.

Officials cited several incidents, including a recent fire on a bus on I-66, in which the system had allowed emergency crews and highway officials to respond rapidly to emergencies. The system has "dramatically improved" the way such incidents are handled, Farley said.