The Virginia highway department has started increasing the duration of the red lights on the computerized ramps leading to Shirley Highway and I-66, according to highway officials.

The lights, which have been the focus of a protracted controversy, are a key part of a $26 million traffic control system that went into operation last week on the two heavily used highways. Officials said the longer red lights should further improve traffic flow.

The lights initially were set at a 4-second cycle, with a 2 1/2-second red light followed by a 1 1/2-second green. Only one car is supposed to enter the highway when a light turns green. Officials said they began with the short cycle to give commuters time to become familiar with the system.

Now most lights have been shifted to longer cycles. One light -- at a ramp from Duke Street to Shirley Highway (I-395) -- has been set at the maximum 12-second cycle, with a 10-second red light and a 2-second green.

Only one light -- at a ramp from Shirlington Circle and Quaker Lane to Shirley Highway -- is still set at a four-second cycle, officials said. All other lights are operating on cycles between 4 and 12 seconds.

Thomas F. Farley, assistant district engineer for the Virginia Department of Highways and Transportation, said the longer cycles appear to have helped ease congestion on the highways without causing backups on the ramps. Further shifts in timing are expected in the next few weeks, officials said.

In another development, officials said they plan to shut off the new system temporarily on I-66, starting at 10 a.m. Friday, to allow construction workers to replace a damaged coaxial cable. Normal operations are to resume on Monday, officials said. The work is not expected to affect Shirley Highway.