High Occupancy Vehicle: (quoting above poster)
A high-occupancy vehicle lane is a restricted traffic lane reserved at peak travel times or longer for the exclusive use of vehicles with a driver and one or more passengers, including carpools, vanpools, and transit buses. The normal minimum occupancy level is 2 or 3 occupants.
HOV use in Tennessee has been expanded to include motorcycles and "ultra efficient hybrid vehicles". This tripled the number of signs on limited access highways having HOV-specific signage.