It's 10:30 a.m. An unknown number of shoppers, litigants and other downtown visitors have overstayed their parking limits. It's Martha Manriquez's job to find them.

Violators and enforcers have been playing a cat-and-mouse game since the dawn of the horseless carriage. Enforcement now has some high-tech tools that tip the contest in their favor.

Manriquez is able to cruise the streets of downtown Napa at 15 mph while a roof-mounted camera on her white Jeep captures license plate images that are fed to a dash-mounted computer.

When an overtime violation is detected, the computer's normal sound, a chirp, becomes a gurgle. Manriquez has nailed another one.

Or maybe not. During a reporter's ride-along last week, Manriquez prepared to write numerous tickets only to discover juror or handicapped placards inside the vehicle.

Others escaped a $25 overtime ticket by observing the letter, if not the spirit, of the law: They had moved their vehicle a few inches, nullifying Manriquez's two-hour clock.

"There is no law that says you have to move a whole block, which is why they can get away moving a few inches," said Manriquez.

This is OK with her. At least "they know they're being watched," she said. Besides, the odds are good that someone who games the system will eventually slip up. Then she'll get 'em.

Manriquez has been enforcing downtown parking limits for the past year. She works the streets while a second officer, using old-fashioned chalk to mark tires, operates in the garages and surface lots.

She likes her job, she says, because she gets to interact with mostly nice people. Sensing her good humor, people ask her for directions, talk about the weather and sometimes even apologize for parking too long.

Only a few -- maybe three over the past 12 months -- have used rough language. "It hurts," she said. "I was warned at the beginning not to take it personally."

Starting her day shortly after 8 a.m., Manriquez had captured the video images and exact locations of 295 vehicles in the first hour and a half. One vehicle was ticketed for staying too long in a 20-minute zone.

Setting out from City Hall at 10:30, Manriquez would write nine tickets over 90 minutes. One violated a 20-minute zone, another ignored hash marks and occupied two spaces. The rest overstayed the two-hour limit.

Manriquez got her first gurgle on Second Street next to the Old Courthouse. Using global positioning system technology, her computer told her that a van hadn't budged in two hours and 11 minutes.

That's a ticket, said Manriquez, noting that the computer is programmed to give motorists a five-minute grace period.

The computer displays license plate images from her first passby with her second pass. The photos are so clear that she is able to compare the position of the tire stems on the two photos.

As she started to write up the car, a woman came running up the sidewalk. "Wait. I'm coming," she yelled. "I was in court. You guys gave me one last week."

Manriquez paused a few seconds, then wrote "void." This was the van owner's lucky day.

There are limits to her good graces, Manriquez said. "I cannot be wasting all the paper -- void, void, void. It won't look good for me either."

She writes the most tickets near the courthouse. "They get stuck in court," she said. "At the same time, they knew they only had two hours and they parked here."

She ticketed a car two spaces away that carried a "Support Our Troops" decal over the rear bumper. Around the corner on Brown Street, she ticketed another one with the same decal.

Her personal record is 38 tickets in an eight-hour shift. An average day is 15 or 20. She has no quota.

On a pass down Main Street between Second and Third, the computer gurgled four times. Nearly half the vehicles on one side of the street were apparent violators.

One turned out to have moved a few inches and was thus legal. Another carried a juror's placard. A man was sleeping in a third vehicle as a woman walked up, saying she'd been delayed in court. "I'm leaving right now," she said. Manriquez ticketed only one of the four.

Seeing Manriquez at work, a man asked if he could park beyond two hours if he put a "U.S. Department of Justice" sign on his dash. Unloading strawberries from his van, another man volunteered, "I just got here."

When Manriquez came across a new vehicle that didn't yet have plates, her camera scanner was of no use. She got out a chalk stick and left a mark.

Passing the city lot at the southeast corner of Randolph and Second, she made sure that each of the vehicles along the street had the required "reserved" marker. The city sells this row, charging $30 a month for a space.

Besides chirps and gurgles, the computer also makes a third sound that designates a scofflaw, someone with five or more unpaid parking tickets.

She found a scofflaw on Main Street, but the vehicle was lawfully parked. On Tuesday, city staff will be asking the City Council for authority to put a disabling boot on the tires of scofflaws whenever they are discovered.

As Christmas comes closer, enforcement becomes more lenient, Manriquez said. "We go easy on them. They're doing their shopping," she said.