Virginia highway officials, gearing up for next week's start of a long-debated new traffic control system, held out a promise yesterday for commuters on Shirley Highway and I-66: 2 1/2 seconds is the longest any signal light initially will stay red at the highways' heavily used entrance ramps.

"We will be starting it in its most nonrestrictive mode," David Gehr, Northern Virginia chief for the Virginia Department of Highways and Transportation, said at a news conference. Because of the short time the lights will be red, officials said, drivers are unlikely to face backups at the ramps.

The long-delayed $26 million system, which has stirred widespread protest from commuters and politicians, is scheduled to go into operation at 10 a.m. Wednesday on Shirley Highway (I-395) between Springfield and the 14th Street Bridge and on I-66 between the Capital Beltway and the Theodore Roosevelt Bridge.

Although the system encompasses a far-flung assortment of computerized and electronic apparatus, the controversy has focused on one key part: the new signal lights that have been installed at 20 ramps along Shirley Highway and at seven entrances to I-66.

To help allay commuters' worries and lessen initial confusion over the lights, officials said they plan temporarily to limit the signal lights to a red-green cycle lasting no longer than 4 seconds: a 2 1/2-second red light followed by a 1 1/2-second green. The signals do not include yellow lights.

Under traffic rules drawn up for the new system, only one car will be permitted to proceed past a signal during each cycle.

Highway officials have argued that the lights will help even out the flow of traffic on the roadways, helping to bring about shorter trips, less congestion and fewer accidents. "We believe that the majority of motorists will have some benefit," Gehr said.

But critics, including Alexandria officials who went to court in an unsuccessful attempt to block the system, have charged that it may cause backups at ramps, congestion on nearby streets and delays for commuters in close-in suburban areas.

The Virginia State Police said drivers who violate the new system may be cited for failing to obey highway signs, a charge carrying possible fines and court costs of $35 to $45. "We're not going to start off with a real strong enforcement effort," said 1st Sgt. Dennis W. Robertson, but he warned that police may crack down if flagrant violations occur.

As commuters and highway technicians become familiar with the new system, officials said, the signals' red-green cycles will gradually be lengthened. The longest cycle is expected eventually to last 12 seconds, with the lights staying red for 10 seconds and green for two.

The longer cycles may cause delays for some commuters at the ramps, officials said. But they contended that these delays will not last more than about two minutes, and they said the waits will be offset by faster trips on the highways.

The new system is expected to save commuters up to 2 1/2 minutes for each mile of a trip, officials said. Quicker rides are most likely, they said, during what are known as the "shoulders" or "fringes," times just before and just after rush hour.

Recent reactions appear mixed. "I'm pretty sure they'll make it work right, considering they've spent $30 million," said Shiva K. Pant, Fairfax County transportation director.

But Charles E. Kenyon, Alexandria's transportation chief, expressed concern about possible backups at ramps and increased congestion on local streets after the signals' cycles are extended. A Shirley Highway ramp at Shirlington is a key issue, he said. "That's the first place where we're going to see a problem."