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When he’s working a carpool enforcement detail, CHP Officer Brandon Correia will scout out a place to lurk in the center median of the highway, waiting to spot scofflaws. On Highway 4, On Highway 4, where the carpool lanes don’t allow toll-paying solo drivers, cheaters are easy to spot.

“It’s really simple,” Correia said, even when there’s a dummy in the passenger seat, as was the case earlier this month when an officer on Highway 4 nabbed a motorist using a mannequin.

On a recent weekday, Correia had barely parked before he saw his first offender: a 20-year-old Stockton resident who told the officer he didn’t realize what days of the week the carpool lane was active. The man, who declined to give his name, said it was his first day working at a new job in Martinez, where he was directing traffic for a construction project.

Correia didn’t cut the man any slack: He drove away with a $490 ticket.

Caltrans estimates carpool cheaters can make up as much as 39 percent of the cars in a diamond lane during peak commute hours, although the rate fluctuates widely depending on the time and location of the count. Local transportation experts estimate the average at 19 to 24 percent.

But CHP officers can only be in so many places at once, which is why regional transportation planners are looking to other methods — from smartphone apps that can confirm the number of people in a car to laser-guided surveillance cameras — to catch HOV-lane cheaters. If the technology works — and that’s still a big “if” — it promises to unclog crowded freeways by getting more people through lanes that already exist, while at the same time reducing carbon emissions. And it is far more cost-efficient than spending billions on new infrastructure, said Joe Rouse, Caltrans’ chief of systems operations.

“We obviously can’t continue to build our way out of congestion,” Rouse said, “so we have to make the most of the system we have.”

If just 3 to 5 percent of commuters would switch to carpooling, everyone on the road would see a 50 percent reduction in congestion, said Andrew Fremier, the deputy executive director of operations for the Metropolitan Transportation Commission, which oversees several toll lanes in the Bay Area.

“That is the overarching reason all of these techniques are important to us, because you’re trying to impact a small number of people,” Fremier said. “With these small improvements, you see dramatic benefits.”

Last year, the commission embarked on an experiment to use new technology to spot HOV-lane scofflaws in a three-week pilot program on Interstate 880. The pilot tested three companies’ claims that their systems could automatically detect cheaters. The companies, Indra, Conduent and TransCore, each use methods that combine surveillance cameras with infrared and laser technology to snap photos of vehicles as they pass. The companies’ software then applies algorithms to process the photos and determine whether the shapes inside the car are humans.

Between 78 and 88 percent of the time, the companies succeeded, the MTC’s Linda Lee told commissioners at a meeting in November to discuss the results. Roughly 5 to 6 percent of the time, the systems gave false positives, inaccurately identifying a vehicle as a solo-occupancy car when it actually had the requisite number of people inside. Lee said staff estimated that could result in about 50,000 drivers each month on I-880 alone receiving hefty fines for a violation they did not commit. Or it would cost the commission about $3 million each year in pay for people to manually review the images before the citations were sent out, she said.

The cost to review images would be on top of setting up the wiring and other infrastructure needed to install and power cameras and related computer equipment along the freeway, Fremier said, with each installation estimated to cost $1.5 million for a single location. When completely built out, that cost could reach hundreds of millions of dollars, according to the MTC, not to mention the ongoing thousands in annual maintenance.

“While we’ve learned the camera systems are getting dramatically better all the time,” Fremier said, “the caution is how much investment in the infrastructure you need to make this system work.”

So, while the technology improves, the MTC is turning its attention to other technology-based solutions that may not require such huge start-up costs. In April, the commission directed staff to explore several smartphone applications that promise to encourage carpooling, though it’s unclear to what extent the apps could be used to hold violators accountable, and they would only work in toll lanes.

Part of the MTC’s and Caltrans’ long-term strategy, however, is to eventually convert all diamond lanes in the Bay Area to toll lanes, where carpools can ride for free and solo motorists can pay to drive in the lanes, a strategy Caltrans’ Rouse said makes the best use of the lanes by ensuring they never sit empty.

The carpool apps — from Cambridge Transportation Labs, Scoop, Waze, GoCarma, Hytch Rewards, RideFlag, Seamgen and TKLABS — work essentially by counting the number of smartphone signals in a car and using that as a proxy for the number of people. The apps can determine when two phones are always together, indicating they belong to the same person, said the MTC’s Pierce Gould at the April commission meeting.

Some apps take another approach, requiring only one person to have a smartphone. The app uses the phone’s camera to verify the number of occupants at the start and end of the trip, with the images stored only momentarily on the device but never sent to the cloud.

In theory, carpools could be required to use them at some point. But that won’t be soon, Gould said, and for the foreseeable future, they would serve mostly as an incentive: The data could be used to confirm eligibility for discounted or free tolls, replacing the FasTrak Flex tag motorists currently must use to get a carpool discount.

But with these apps come serious privacy concerns, and that’s something Gould said the MTC would explore in detail when the pilot program is launched sometime next year.

“Privacy is a really important issue,” he said.

The year-long program would ask about 500 carpools to participate in exchange for a monetary reward. The carpools would download an app and test it in an express lane, allowing the MTC to verify the app’s accuracy, its vulnerability to cheating and any areas where privacy protections are weak, Gould said.

“The goal would be to gauge public awareness, to verify the accuracy of the app system and to gain experience with the tool itself,” Gould said.

In the meantime, the commission is doubling down on enforcement. In April, it approved an extension of a contract signed in 2018 to continue targeted enforcement along Interstate 80, building off a 2017 contract for carpool patrols on Interstate 680. Interstate 880 will be added in 2020, when express lanes open there. The contracts together totaled $6.7 million from the start of the contracts until 2020 for I-80 and 2021 for the other corridors.

The additional enforcement already has yielded results. Carpool citations in the Bay Area increased 80 percent last year, from 18,476 in 2017 to 33,296 in 2018, according to data from the CHP. That’s a big jump, considering the citation rate was relatively stable from 2012 to 2017, with each year ranging from just over 18,000 to just under 20,000.

“The uptick is a direct result of those contracts,” said CHP Sgt. Robert Nacke.

But using officers to police carpool lanes is a limited tool, Fremier said. It provides a deterrent to cheating when the officers are present but almost none when they are not, he said. Each interaction with violators takes between 20 to 30 minutes, Correia estimated, meaning that at its most efficient, an individual officer can only nab two to three violators per hour.

For comparison, in a 2018 report, Caltrans counted roughly 56 vehicles per hour violating carpool rules on Highway 4 during a three-hour span in the afternoon commute on one day in October 2017. On Interstate 80, during the afternoon commute in December 2017, Caltrans counted 406 violations per hour.

“It’s a great tool when it’s there,” Fremier said of the CHP patrols, “but it’s very focused and very isolated where the CHP can provide their resources.”

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On a recent weekday, Sandy Bell, of Pittsburg, was late dropping off her rented Camaro. There was someone who appeared to be texting and driving in front of her, she said, so she scooted into the carpool lane to get around the slow driver. Officer Correia didn’t buy her excuse.

“I’m not going to lie,” Bell said, “I know it was wrong. He got me.”