A $500,000 high-tech traffic detection system installed a year ago at 22 intersections along Route 7 does not work properly at 20 of them, and the state has reverted to using the system it was trying to replace, Virginia transportation officials said.

After receiving money from the federal government last year, the Virginia Department of Transportation put up 67 video cameras at intersections between Leesburg and Tysons Corner. They were intended to be a 21st century solution to improving the flow of traffic on one of Northern Virginia's most heavily traveled highways, where commuters crawl from stoplight to stoplight between the new suburbs of Loudoun County and closer-in job centers.

The cameras were designed to detect oncoming cars and automatically relay that information to an electronic controller, which would change the timing of traffic lights accordingly.

In theory, the cameras would detect where cars were coming from and drivers could continue on their way, instead of stopping at one light after another. In reality, many drivers found themselves sitting at lights, waiting and waiting some more for them to change.

"We found some problems," said Dennis C. Morrison, VDOT's Northern Virginia administrator. "A car may have come to an intersection and it wasn't detected."

So officials have reverted to the old "loop detectors" -- long, rectangular metal detectors built into the pavement that are designed to detect cars.

Aside from replacing what they thought was a less-reliable system -- the loop detectors don't respond to motorcycles, among other drawbacks -- the cameras promised to save money on upkeep. Loop detectors cost less to install but require constant maintenance because of problems caused by weather and damage from snowplows and utility crews.

Route 7 is used by about 72,000 drivers a day. A former country road, it runs from Alexandria to Winchester and has been widened and reworked several times as the area around it has boomed.

One of the worst sections is between Leesburg and Tysons Corner, where the main part of the highway widens to as many as six lanes. Traffic halts every weekday, during the morning and afternoon rush hours, and every weekend as residents run errands to new shopping centers.

Officials have nearly run out of ways to lessen these daily jams and had hoped the cameras would make a difference.

"They painted a fairly optimistic picture," said Rep. Frank R. Wolf (R-Va.), who secured the funds for the project. "I feel a little used, because I've always cooperated with VDOT over the years and they didn't tell me [about the problems]. And I've been telling people they've synchronized the lights."

The federal government has allocated an additional $1 million to install cameras on Route 50 and Route 28. Morrison said he will consult with Wolf and wait for the technology to improve before putting up those cameras.

He also said it was his fault that Wolf wasn't told about the problems sooner.

Morrison said problems started at all 22 intersections on Route 7 almost as soon as the cameras -- and the sun -- went up.

"The sunlight at certain times of day sets a glare to the point where the video can miss a car," Morrison said, noting that Route 7 runs east and west and sunrises and sunsets are particularly problematic.

The cameras didn't work much better in the dark, and some took as many as two months to calibrate. Many of the intersections are also so wide that VDOT hasn't been able to position the cameras to catch traffic all the way across the highway.

VDOT officials said that they have minimized some of these problems and that cameras along the main lanes of Route 7 are being used. But they said the loop detectors are needed on side streets and that they're relying solely on the cameras at only two of the 22 intersections.

"We're having problems and working with the manufacturer," Morrison said. "I'd like to think some of these problems can be worked out. If they can't, we'll just have to continue to use the loop detectors."