From a look at Rob Sohlberg's E-ZPass charges, you'd think he was a truck-driving man.

When the Baltimore motorist opened his most recent monthly statement of electronic toll charges, he found three charges of $4 for his wife's trips through the Fort McHenry Tunnel in her Toyota Matrix. He was charged $8 for a tunnel trip in his Toyota Camry.

The correct toll for a passenger vehicle at the tunnel is $2.

What Sohlberg discovered was that the axle-counting device at the tunnel had counted his wife's car as having three axles and his as having five.

According to the Maryland Transportation Authority, which runs state toll facilities, the Sohlbergs weren't the only E-ZPass tunnel customers overcharged.

Kelly Melhem, a spokeswoman, said a malfunction of the axle-counting device appears to have resulted in "a couple thousand" overcharges at Lane 3 of the northbound McHenry tunnel in December -- the second such occurrence in Maryland recently. She said an October problem with a similar device led to about 1,000 overcharges at the Bay Bridge.

She said both devices, called treadles, have been replaced, and the state has stepped up maintenance and monitoring in an effort to prevent such problems.

"It's not that it's defective equipment. It's just nearing the end of its life span," Melhem said.

E-ZPass is the electronic toll collection service used by transportation agencies from Maine to Virginia and as far west as Illinois. Maryland has been part of the system since 2001 -- using it to speed traffic through tunnels and bridges and the state's toll plaza on Interstate 95 northeast of Baltimore.

But the Sohlbergs' experience adds to the growing evidence that E-ZPass is not perfect. That is why, Melhem said, it is a good idea for subscribers to read their statements. She said the Sohlbergs have received credit for the overcharges.

The authority is reviewing all transactions that took place while the treadle was malfunctioning and will credit accounts when appropriate, Melhem said. She said that while E-ZPass gets the charges right more than 99 percent of the time, its electronic sensors and cameras can on occasion get them wrong.

"It can be fooled, but that's not something that happens often," she said. "When you do have electronic systems, you are going to have random technology glitches."

One way E-ZPass can be fooled is when the treadles -- spring-mounted metal monitoring devices embedded in the road -- wear out, Melhem said. The system also can miscount axles when one vehicle goes through the toll collection lane while tailgating another. When that happens, sometimes the axle-counting device doesn't recognize that two vehicles have passed through.

In some cases, that can result in a driver being charged for the tailgater's toll, Melhem said. But lest anyone be tempted to use tailgating as a strategy to beat tolls, she added that the system can use license plate photos to identify a vehicle -- and the tailgater could be socked with the extra charge.

"We do ask our customers to obey posted speeds in the lanes and not to follow other vehicles too closely," she said.

James A. Crawford, executive of the E-ZPass Interagency Group in Atlantic City, said he believes tailgating, rather than malfunctioning equipment, is more often the reason for axle miscounts. But he doesn't think most tailgaters are doing so to beat tolls.

"More likely they're just aggressive drivers. It's a highly dangerous way to save 50 cents or even $2," he said.

Crawford said the technology used for "separating" one vehicle from another can also be affected by ice storms or heavy snow.

Generally, the software that runs the system can be programmed to detect when a disproportionate number of vehicles with more than two axles are passing through toll lanes. When that happens, Crawford said, the system sends an alert to operators that something might be wrong with the equipment.

Like Maryland officials, Crawford insisted that E-ZPass performs at better than a 99 percent accuracy level systemwide.

Maryland E-ZPass customers with questions about their accounts can call 888-321-6824.

michael.dresser@baltsun.com