Virginia's controversial plans to start operating a computerized $26 million traffic control system today on Shirley Highway and I-66 were threatened with an 11th-hour delay yesterday because of accidental damage to communications and power cables.

A spokeswoman for the Virginia Department of Highways and Transportation said the agency had dropped its plans to start the system on both roads at 10 a.m., but would seek to put it into service on Shirley Highway (I-395) before this afternoon's rush hour. Prospects for launching the system today on I-66 were termed unlikely.

"There's still a lot of uncertainty about the extent of the damage out there," said Lynda J. South, spokeswoman for the department's Northern Virginia office. "We are going to make every effort to make some temporary repairs."

The long-delayed system, which includes signal lights at entrance ramps, was left with insufficient electric power and inadequate computer and closed-circuit television transmissions because of the two accidents, officials said. Parts of the system already have been delayed for two years.

"We don't think it will function properly," Gene Hull, an assistant district engineer for the highway agency, said as officials sought to determine the impact of the damage. "The information that is coming through is all fouled up at the present time."

Officials said they would seek to patch the damaged cables after electric power is restored. If the system is repaired, thousands of drivers will get their first glimpse at new signal lights, traffic rules and computer gadgetry.

The federally financed system is to undergo a shakedown period of several weeks to allow commuters and technicians to become familiar with it. "There's going to be confusion initially," said Thomas F. Farley, assistant district engineer in charge of the system.

Officials said the damages that are expected to delay initiating the system were caused by construction crews currently at work on a $1.5 million project to widen ramps at the interchange of I-66 and the Capital Beltway. The project, being carried out by Moore Brothers Co. Inc. of Verona, Va., is not related to the traffic control system.

On Friday, company and highway officials said, workers employed by an electrical subcontractor accidentally severed an electric power cable linked to the traffic control system. The cable is expected to be repaired by today, officials said.

On Monday, officials said, a front-end loader operated by Moore employes accidentally crushed a plastic conduit carrying communications cables. Because of the previous power loss, officials said they could not immediately assess the damage to the transmission lines. However, the incident was expected to result in electrical interference throughout the system.

Highway officials have contended that the new system will eventually result in quicker trips, less congestion and fewer accidents. Opponents have countered by warning of possible delays at the entrance ramps, backups in surrounding neighborhoods and increased congestion on residential streets.

Advocates and critics both say it may take some time for the system's impact on rush-hour traffic to become clear. "It's not going to happen like magic all at once," said David Gehr, Northern Virginia chief for the state highway agency. Some local officials plan to monitor traffic closely.

The system is to encompass an 11.5-mile section of Shirley Highway between Springfield and the 14th Street bridge along with a 10.1-mile stretch of I-66 between the Capital Beltway and the Theodore Roosevelt Bridge. The ramp signals are to operate from 5:30 a.m. to 9:30 p.m. on weekdays.

Although some commuters may be delayed at ramps, highway officials have said, these initial waits are expected to be offset by quicker trips on the two highways. Traffic is likely to move faster, officials said, because there will be less weaving, fewer abrupt slowdowns and a decrease in accidents, especially rear-end collisions.

Initially, highway officials plan to run the ramp signals on a 4-second cycle, with a 2 1/2-second red light followed by a 1 1/2-second green. Only one car in each lane will be permitted to drive past a signal during each cycle. Later, officials said, the cycles will be lengthened.

Eventually, some signals are expected to operate at the system's maximum 12-second cycle, with a 10-second red light and a two-second green. Others may be set at shorter cycles. Even with 12-second cycles, official have estimated, commuters should not be delayed on a ramp for more than about two minutes.

Officials have cautioned that drivers still must yield to oncoming highway traffic after they pull off a ramp on a green light. The system is not designed to detect gaps in highway traffic, officials said. Such gap systems have proven unworkable elsewhere, officials contended.

Police have warned that drivers who violate red lights at ramps may be cited for failing to obey highway signs, a charge carrying possible fines and court costs of $35 to $45.

According to highway officials, the system includes a "failsafe mechanism" aimed at preventing excessive backups at ramps. If a ramp becomes jammed, the system is designed to speed up the signals' cycles. If backups remain, technicians may switch to an uninterrupted green signal, officials said.

The $26 million system has also included other measures, such as widening sections of Shirley Highway, installing electronic gates at the highway's reversible express lanes, and setting up closed-circuit television and other devices to speed responses to accidents by emergency workers