Florida state regulators are investigating the Hertz Corp.'s use of electronic toll-paying systems, after five customers told officials they were not properly notified that the vehicles they rented in Florida were registered with a company called PlatePass, according to the Sun Sentinel.

PlatePass has a contract with Hertz to provide electronic tolling services in its 300,000 vehicles nationwide.

But some customers said the electronic tolling system activated automatically and charged them tolls without their knowledge. Others complained that they continued to be charged a service fee even on days when they didn't use toll roads. Hertz declined to comment on the complaints or the investigation.

Almost all major rental car companies can electronically collect tolls in Florida and other states. With the service, customers can use cashless or high-speed express lanes and bypass long lines at toll booths.

But many consumers are outraged by the extra charges. In March, two private lawsuits seeking class-action status were filed against Hertz, PlatePass, and its parent corporation. Customers who filed suit were upset about what they said were limited disclosures.

State regulators settled a case against Avis Rent A Car System, which also owns Budget, last year over allegations involving its use of PlatePass.

A representative for the Consumer Travel Alliance advocacy group in Washington, D.C., said the rental car tolling issue will evolve as highways continue to add cashless express lanes and switch to unattended toll gates. No standard state or federal regulations exist for these systems.

Bob Barton, president of the American Rental Car Association, said the industry continues to fine-tune its electronic tolling services as the technology evolves.