High occupancy/toll (HOT) lanes are lanes that are open only to high occupancy vehicles and those that pay a toll to use them. In many HOT lanes, single-occupant vehicles (SOV) are required to pay a toll and high-occupancy vehicles (HOV) either pay a reduced toll or are charged no toll at all. Access to HOT lanes is granted as a privilege to certain classes of users (e.g. buses, high occupancy vehicles, hybrid vehicles) or based on a fee, while adjacent or parallel "general purpose" lanes or facilities remain free. Thus, HOT lanes almost always represent a choice to users whether or not to use them.