The purpose of the HOV system in Los Angeles County is to enhance mobility for all County residents and visitors by providing a variety of transportation options. In Los Angeles County, the HOV system includes freeway HOV lanes, HOV access ramps, park-and-ride lots and transit stations along HOV corridors.
The HOV system gives travelers who carpool, vanpool or ride the bus more reliable travel on freeways. The availability of this more reliable travel provides a strong incentive for even more people to choose to leave their cars at home and travel by carpools, vanpools or buses.
Today, the State of California has 1410 HOV lane miles, representing approximately 2.78% of the state highway system's total lane miles. Over 36% of these HOV facilities (513 lane miles) can be found in Los Angeles County, making it one of the largest HOV systems in the country. 
Historical Development of HOV in Los Angeles County
The first HOV facility in Los Angeles County was opened in 1973 as a bus-only facility known as the El Monte Busway. Operating along the I-10 freeway corridor between El Monte and downtown Los Angeles, the El Monte Busway was opened to carpools of three persons or more in 1976. Recent changes in California state legislation will result in a reduction in the carpool size requirement on the El Monte Busway to two persons or more for times outside of weekday peak periods.
Despite the success of the El Monte Busway project, the further expansion of the Los Angeles County HOV system was limited between 1976 and the early 1990's. During this period, a "Commuter Lane" was added to the SR-91 Freeway as the only new HOV lane project in the County. The SR-91 Commuter Lane, which opened in 1985, provided a eastbound travel lane for carpools of two or more persons.
The Intermodal Surface Transportation Efficiency Act (ISTEA) was enacted in 1991 changing many federal requirements regarding the planning and programming of the transportation infrastructure. ISTEA promoted innovative ways to address transportation system deficiencies and to improve mobility and air quality in major metropolitan areas. HOV was identified in ISTEA as an appropriate transportation solution for addressing these problems.
In 1992, the Los Angeles County Transportation Commission (LACTC), which was one of the predecessor agencies of Metro, adopted the "Carpool Lane Plan" for Los Angeles County. The plan detailed a system of HOV facilities to serve Los Angeles County and established priorities for HOV system development. In 1992, LACTC and Caltrans also signed a Master Cooperative Agreement for the development of the County HOV system, signaling the start of rapid HOV system expansion.
In June 1993, there were 58 lane miles of HOV lanes in Los Angeles County. By the following year, an additional 73 lane miles of HOV lanes had opened on County freeways, including the Century Freeway (I-105) carpool lanes. In 1996, the Harbor Freeway (I-110) Transitway had opened for transit bus, vanpool and carpool users. By June 1997, the Los Angeles County HOV system had reached 269 lane miles. Between June 1997 and January 2010, 244 lane miles of HOV lanes were added to the County HOV system, bringing the total system to 513 lane miles.
Existing HOV System in Los Angeles County
Today, the Los Angeles County HOV system is carrying more people than any other HOV system in the United States, and is one of the few HOV systems in the country that has been able to sustain a growth in carpools.
The Los Angeles County HOV system is part of a larger regional HOV system that serves the five counties of the Los Angeles metropolitan area (Los Angeles, Ventura, Orange, San Bernardino, and Riverside). The Los Angeles metropolitan area HOV system covers over 960 lane miles, representing over 68% of the HOV lane miles in the entire State of California.
The entire County HOV system is open to only HOV traffic, 24 hours a day-seven days a week. With the exception of the El Monte Busway, all Los Angeles County freeway HOV lanes permit carpools with a minimum occupancy of two persons. Changes in California State legislation allow Ultra Low Emission Vehicles (ULEV's) to use HOV facilities throughout the state, regardless of occupancy.
On average, a HOV lane in Los Angeles County accommodates 1,300 vehicles or 3,300 people per hour during peak periods, and the HOV system serves approximately 331,000 vehicle trips or 780,000 person trips per day. 
In addition to the HOV freeway lanes, the system includes freeway-to-freeway HOV direct connector interchange ramps, direct freeway HOV lane to transit terminal bus ramps, park-and-ride lots, freeway to park-and-ride connector ramps, direct freeway HOV lane entrance and exit ramps and freeway ramp meter HOV bypass lanes.
Metro, in cooperation with Caltrans, is in various stages of planning, design and construction for additional HOV facilities for Los Angeles County. The Los Angeles County HOV System Status Map illustrates the status of HOV projects currently programmed by Metro, in addition to the existing HOV system in Los Angeles County.
Currently, there are a total of 513 HOV lane miles completed to date and 132.2 HOV lane miles currently in construction, design or planning on six major corridors. This includes 43.4 HOV lane miles currently in construction or going to construction (I-10, SR-60, the I-405 Design/Build project and the I- 5/SR-14 Direct Connector), 52 HOV lane miles currently in design (I-5 and I-10) and 36.8 HOV lane miles currently in the planning stage (I-5, SR-14 and SR-71). Over the next year, we anticipate opening 22.6 additional HOV lane miles.
 California Department of Transportation, “ Data Integration and Reporting Branch (DIRB) State Highway Mileage and Other Data." November 2009
 Caltrans District 7, " 2008 HOV Annual Report - Executive Summary ", January 2009