Many commuters appeared puzzled by the new red and green traffic signals that seemed to flash as quickly and randomly as lightning on 19 Shirley Highway (I-395) ramp entrances and seven I-66 entrances yesterday.

"If you're getting ready to go on a highway, you usually speed up," said Jose Campos of Arlington as he stopped at the signal at the Glebe Road ramp on I-66.

Campos, like hundreds of others on the highway, said he did not understand exactly what he was supposed to do when he neared the unfamiliar flashing signal.

In spite of the confusion, highway officials said traffic on two of Northern Virginia's interstate highways flowed smoothly on the first full day of operation of the new $26 million computerized traffic control system.

While not completely free of glitches, the controversial traffic system, which includes signal lights at the ramps on Shirley Highway and I-66 did not cause backups or fender-benders on the ramps or congest residential roads nearby, as many had feared it would.

"The traffic moved smoothly," said Lynda J. South, a spokeswoman for the Virginia Department of Highways and Transportation. "There were a few backups, but no accidents."

"Things are much the same," said Norman F. McCart, an Arlington patrolman. He said the only problem was caused when motorists stopped short of the white lines that trigger the signal changes.

A Washington Post test of one commuter route -- from I-95 at Fraconia Road, just beyond the Capital Beltway, to just before the 14th Street Bridge -- showed that drive time was shortened by four minutes, from 30 minutes before the new system was implemented to 26 minutes with the signals. The driver also found that the traffic was less stop-and-go, moving more fluidly.

The signals, most of which switched from red to green in 4-second cycles yesterday, were programmed to allow one automobile at a time to merge with highway traffic. But some of the lights were not turned on, highway officials said, because either the traffic was too light or too heavy. During heavy traffic, the lights are not turned on to avoid causing backups on ramps to spill into residential roads.

State police said dozens of other drivers, including one who received a written warning at the northbound Shirlington Circle ramp onto I-395, chose to ignore the red-flashing light. Beginning July 1, state police will issue $35 to $45 tickets for the violation.

The federally funded traffic control system, which has been delayed for two years, uses sensors buried under the ramps and television cameras posted near the highways to read and respond to traffic patterns.

Highway officials say that the signals will mean a more even flow of traffic, quicker trips, fewer drivers weaving between lanes and fewer accidents -- though there may be a delay of up to two minutes on the ramp.

Opponents, including some Alexandria and Fairfax officials, argued in a court case, which they lost, that the ramp lights gave an unfair preference to motorists from outlying areas because it only delayed those entering the highways close to the District.

Charles Kenyon, Alexandria traffic division chief, said yesterday that even though the traffic moved smoothly yesterday, he "still wouldn't have spent a penny on it."

A resident near Shirlington Circle, James A. Zurawski, said he didn't approve either. "The people who live closer in get zapped. We spent more money to live closer into the city and it's us who have to sit on the ramps."