Virginia's long-debated $26 million traffic control system went into operation yesterday on Shirley Highway and I-66, causing confusion for some commuters, short backups at several entrance ramps and one minor accident.

The computerized system, which includes signal lights at the ramps, was switched on at midmorning after last-minute repairs to damaged electrical cables. Officials previously had said that the damaged cables might delay the system's start.

Highway officials said backups occurred at some ramps because drivers stopped too soon to trigger devices implanted in the ramps that switch the lights from red to green. Cars are supposed to stop near white pavement markers to make the lights turn green.

"They have to pull up to that stop bar," said David Gehr, Northern Virginia chief for the Virginia Department of Highways and Transportation. "It's a concern that we're just going to have to live through and get everybody educated."

State police said one accident appeared to have been caused partly by the new system. The incident occurred on a ramp leading from Duke Street to Shirley Highway. WTOP traffic reporter David Statter said a car and a truck that had stopped at a red light were sideswiped by a car that entered the ramp behind them.

Despite these mishaps, officials said, the system ran relatively smoothly and appeared unlikely to cause major delays for rush-hour commuters this morning. "The motorists seem to be accepting it," Gehr said.

American Automobile Association spokesman Doug Neilson called on commuters to show caution. "Take your time. Give it a chance. Before you condemn it, let's see if it can work," he said.

Highway officials have contended that the system eventually will lead to faster trips, less congestion and fewer accidents. Opponents, including Fairfax County and Alexandria officials, have charged, however, that the ramp signals may cause delays for commuters and traffic tie-ups on local residential streets. The two jurisdictions failed in a lawsuit to block start-up of the system.

Proponents and critics both say it may take at least several weeks to gauge the system's impact on rush-hour traffic. Officials launched the system under an initial plan aimed at minimizing possible disruption for commuters.

The signal lights have been set at a 4-second cycle, with a 2 1/2-second red light followed by a 1 1/2-second green. Each cycle is triggered when a car approaches an entrance marker. Eventually, officials said, the cycles at some ramps will be extended 12 seconds -- a 10-second red light and a 2-second green.

Only one car in each lane is supposed to drive past a green light during each cycle. The system is in effect 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.

First Sgt. Dennis W. Robertson, the state police commander on the two highways, said most drivers appeared to comply with the new rules, despite some initial confusion. He noted several instances, however, in which two or more cars illegally pulled out of a ramp at one time.

"They just see the green and start rolling," Robertson said. "Several try to go through." Police have warned that drivers who violate the new rules may be cited for failing to obey highway signs, a charge carrying possible fines and court costs of $35 to $45.

The system's start-up had been thrown into doubt when construction accidents at a highway-widening project damaged two key cables. On Tuesday night, however, workers repaired a severed electrical power cable. When power was restored, the system was found to operate despite damage to the other cable, which transmits computer data.

"We thought we were going to have a lot more problems," said Bill Domjan, an engineer for Sperry Corp., the highway department's chief consultant on the project. "We didn't think the system was going to work this well."

Nevertheless, officials said, the damaged 1,900-foot section of coaxial cable must soon be replaced to prevent deterioration.