The difference between a car and a truck is obvious to most people, but it’s not so obvious to the cameras used in San Francisco’s FasTrak electronic toll system.

Nearly 600 car drivers in the city have challenged toll charges they received via the system, claiming that they were charged $12.50, the fee for a truck, rather than the $3 charge for a car.

According to the San Francisco Chronicle, the Metropolitan Transportation Commission, which oversees the FasTrak program, is blaming the problem on dirty lenses.

Randy Rentschler, a spokesman for the commission, told the Chronicle that sometimes the lenses become coated with grime from exhaust fumes or are smeared by rain, thus rendering them unable to tell the difference between a car and a truck.

The exact number of overcharges is unknown. And, because drivers may not notice the overcharges on their FasTrak bills, the number may never be known.

The FasTrak system collects more than 3 million tolls per month in the Bay Area.