High-occupancy vehicle lane

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 at least one passenger, 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 30 mi (48 km) long, extends from Washington, D.C., to Dumfries, Virginia, and has two reversible lanes separated from the regular lanes by barriers, with access through elevated on- and off-ramps. Three or more people in a vehicle (HOV 3+) are required to travel on the facility during rush hours on weekdays.

The second freeway HOV facility, which opened in 1970, was the contraflow bus lane on the Lincoln Tunnel Approach and Helix in Hudson County, New Jersey. According to the Federal Highway Administration (FHWA), the Lincoln Tunnel XBL is the country's HOV facility with the highest number of peak hour persons among HOV facilities with utilization data available, with 23,500 persons in the morning peak, and 62,000 passengers during the four-hour morning peak.

The first permanent HOV facility in California was the bypass lane at the San Francisco–Oakland Bay Bridge toll plaza, opened to the public in April 1970. The El Monte Busway (I-10 / San Bernardino Freeway) in Los Angeles was initially only available for buses when it opened in 1973. Three-person carpools were allowed to use the bus lane for three months in 1974 due to a strike by bus operators, and then permanently at a 3+ HOV from 1976. It is one of the most efficient HOV facilities in North America and was converted into a high-occupancy toll lane operation in 2013 to allow low-occupancy vehicles to bid for excess capacity on the lane in the Metro ExpressLanes project.

Beginning in the 1970s, the Urban Mass Transportation Administration recognized the advantages of exclusive bus lanes and encouraged their funding. In the 1970s the FHWA began to allow state highway agencies to spend federal funds on HOV lanes. As a result of the 1973 Arab Oil Embargo, interest in ridesharing picked up, and states began experimenting with HOV lanes. In order to reduce crude oil consumption, the 1974 Emergency Highway Energy Conservation Act mandated maximum speed limits of 55 mph (89 km/h) on public highways and became the first instance when the U.S. federal government provided funding for ridesharing and states were allowed to spend their highway funds on rideshare demonstration projects. The 1978 Surface Transportation Assistance Act made funding for rideshare initiatives permanent.

Also during the early 1970s, ridesharing was recommended for the first time as a tool to mitigate air quality problems. The 1970 Clean Air Act Amendments established the National Ambient Air Quality Standards and gave the Environmental Protection Agency (EPA) substantial authority to regulate air quality attainment. A final control plan for the Los Angeles Basin was issued in 1973, and one of its main provisions was a two-phase conversion of 184 mi (296 km) of freeway and arterial roadway lanes to bus/carpool lanes and the development of a regional computerized carpool matching system. However, it took until 1985 before any HOV project was constructed in Los Angeles County, and by 1993 there were only 58 mi (93 km) of HOV lanes countywide.