A motor vehicle purchased or used by a county to provide county services shall not operate on gasoline other than ethanol blended gasoline as defined in section 214A.1. The motor vehicle shall also be affixed with a brightly visible sticker which notifies the traveling public that the motor vehicle is being operated on ethanol blended gasoline. However, the sticker is not required to be affixed to an unmarked vehicle used for purposes of providing law enforcement or security.