On this road, they're using e-tags that have a "HOV" switch on them. If you set it to "HOV mode", you're not charged a toll. The tag can't actually know how many people are in the car, so someone could cheat by setting their tag to HOV and driving without anyone else in the car. Hence the actual officers-pulling-people-over enforcement required. (They have readers in the police cars that tell them whether a vehicle's transponder is in HOV mode or not.)
