A high-occupancy vehicle lane (also known as an HOV lane, carpool lane, diamond lane, 2+ lane, and transit lane or T2 or T3 lanes) is a restricted traffic lane reserved for the exclusive use of vehicles with a driver and two or more passengers, including carpools, vanpools, and transit buses. These restrictions may be only imposed during peak travel times or may apply at all times. There are different types of lanes: temporary or permanent lanes with concrete barriers, two-directional or reversible lanes, and exclusive, concurrent, or contraflow lanes working in peak periods. The normal minimum occupancy level is two or three occupants. Many jurisdictions exempt other vehicles, including motorcycles, charter buses, emergency and law enforcement vehicles, low-emission and other green vehicles, and/or single-occupancy vehicles paying a toll. HOV lanes are normally introduced to increase average vehicle occupancy and persons traveling with the goal of reducing traffic congestion and air pollution.
The introduction of HOV lanes in the United States progressed slowly during the 1970s and early 1980s. Major growth occurred from the mid-1980s to the late 1990s. The first freeway HOV lane in the United States was implemented in the Henry G. Shirley Memorial Highway in Northern Virginia, between Washington, DC, and the Capital Beltway, and was opened in 1969 as a bus-only lane. The busway was opened in December 1973 to carpools with four or more occupants, becoming the first instance in which buses and carpools officially shared a HOV lane over a considerable distance.
In 2005, the two lanes of this HOV 3+ facility carried during the morning peak hour (6:30 am to 9:30 am) a total of 31,700 people in 8,600 vehicles (3.7 persons/veh), while the three or four general-purpose lanes carried 23,500 people in 21,300 vehicles (1.1 persons/veh). Average travel time in the HOV facility was 29 minutes, and 64 minutes in the general traffic lanes. As of 2012, the I-95/I-395 HOV facility is long, extends from Washington, D.C.