On the I-15 Express Lanes in San Diego, solo drivers must have a transponder properly mounted 24 hours a day, seven days a week. Solo drivers should not enter the Express Lanes when "HOV Only" is displayed. Carpools with two or more occupants, vanpools, Clean Air Vehicles, motorcycles and transit vehicles drive for free without a transponder. No transponder is needed for qualifying carpools at this time. If qualifying carpools/vehicles have a switchable transponder (FasTrak Flex), it should indicate that there are two or more occupants in the vehicle to avoid being charged a toll. Vehicles using sticker, standard, external and headlamp sticker transponders are charged the full toll regardless of the number of occupants or type of vehicle.