The Minnesota Department of Transportation (MnDOT) is going on offense by testing cameras that in the future could help state troopers pull over and ticket drivers found illegally using special lanes reserved for carpools and solo drivers who pay a fee to use them.

An infrared camera installed this summer on northbound Interstate 35W at Black Dog Road in Burnsville has been snapping photos of drivers in the MnPass lane who pass by a detector that does not register the presence of a valid toll-collecting tag. MnDOT is testing the system to see if it will help law enforcement accurately determine the number of people in a vehicle or if a solo driver is paying to use the lanes.

The system will not be a substitute for human eyes and will not generate and send tickets to offending motorists, said MnDOT spokeswoman Bobbie Dahlke. Troopers will have to visually observe a violation before executing a traffic stop, added Lt. Tiffani Nielson of the State Patrol.

Last August, MnDOT entered into a two-year agreement in which the agency is paying the State Patrol $2.6 million to dedicate six officers to enforce MnPass rules in lanes on I-35W, I-35E and I-394. Since then, troopers have written 3,633 tickets and issued 3,376 warnings, the State Patrol said.

Violators face costs of up to $200.

Starting in August 2018, two additional troopers will be added to the MnPass unit to look for cheaters.

"Violations are a concern for other motorists and legislators," Dahlke said. Installing the system "is one way to curb the violation rate."

MnDOT is currently conducting a study to determine how many drivers illegally slip into the lanes marked with a diamond. But anecdotal evidence shows that as many as 18 percent of drivers on I-35E in the east and northeast metro are cheating. About 7 percent of drivers in express lanes on I-394 between downtown Minneapolis and the western suburbs and on I-35W between downtown Minneapolis and Burnsville are there illegally, according to 2016 estimates.

Dahlke said the much higher rate on I-35E may be because the lanes are only a year old and people are still adjusting to them.

MnPass lanes are designed to keep traffic flowing at 50 mph or greater. Solo drivers pay 50 cents to $8 per trip depending on congestion. But the express lanes bog down when too many nonpaying drivers enter. That defeats the purpose of MnPass, which is aimed at providing congestion relief, and is one reason for testing the new system, dubbed Enforcement Assistance System (EASy). It is part of a $260,000 contract with SRF Consulting of Plymouth.

The system mimics the operation of a two-trooper team, in which one watches vehicles at a MnPass reader site and radios ahead to another squad down the road about a suspected violation. With EASy, a unit tries to read a MnPass tag, and if one is not detected, alerts a camera that takes a picture of the vehicle. The image is passed on to a waiting trooper who can try to determine if there's more than one occupant in the vehicle and make a stop and determine if there is a violation. Motorcycles and buses are also allowed to use the lanes without having to have a tag.

"We have not and will not use the camera as our only source for issuing a citation," Nielson said.

Early returns show EASy has promise, but there are a number of issues to work out before it can go live, Dahlke said. The camera, for example, provides only a side view, meaning images do not show the windshield where a tag would hang or a vehicle's license plate. During daylight hours, "pictures have been very good," she said. But they also have revealed problems that need to be addressed.

"Where do we put the camera? Do we have the right angle and will that give a different image? Did we snap too soon? What happens when the sun goes down at 6 p.m. or snow covers it?" Dahlke said. "There are data privacy issues. Those are things we are working through."

To get answers to those questions and issues, MnDOT recently extended the contract to continue testing through the winter.

Another part of EASy includes beacons that flash when a vehicle without a tag passes by a scanner. That allows an officer upstream from a reader to spot a possible violator, and to spot scofflaws in low light conditions.

MnDOT does not need legislative approval to enact EASy, but would have to determine if it is worth the cost to install it, Dahlke said.

The number of MnPass subscriptions has risen from 27,002 in October 2015 to 38,825 as of this week, MnDOT said.