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
