A high occupancy vehicle (or HOV) is any vehicle with a driver and one or more (or sometimes two or more, or three or more) passengers. The term is used in transportation engineering and transportation planning. When an automobile is used as an HOV, it is often called a carpool, though the term HOV includes buses and vans.
A HOV or carpool may be allowed to travel on special road lanes, usually denoted with a diamond marking in the United States, on which single occupant vehicles are prohibited, called carpool lanes or diamond lanes. In some areas, such as Southern California, the HOV lanes are full-time, while in others, such as the San Francisco Bay Area, they are usable by other vehicles outside of peak hours.
The relative rarity of high occupancy vehicles to single occupancy vehicles in the United States make HOV lanes work. When it is uncongested, an HOV lane can move at full speed even when parallel (non_HOV) lanes suffer delays from queueing at bottlenecks. In theory (and often in practice), an HOV lane moves more people per lane at a higher speed while moving fewer vehicles.
Proponents of HOV lanes say that this is a good return on the laws, paint and signs that an HOV lane requires. Additionally, a single engine carrying multiple passengers uses less fuel per trip, saving money and creating less carbon dioxide and noxious pollutants than if each passenger drove their own car.
In practice, however, the proximity of a slowly moving lane adjacent to the HOV lane and occasional merging slows HOV traffic and thus makes them less efficient than theory would suggest. Multiple or separated HOV lanes can be used to address these issues, at significant costs in terms of space.
Most cities that use separated HOV lanes make them reversible; i.e. usable only by inbound traffic during the morning rush and usable only by outbound traffic during the evening rush. Some say this is a space-economical method of handling high HOV traffic. Critics of this method often cite a 1995 incident in Pittsburgh, Pennsylvania, when a negligent highway employee failed to close the gate preventing access to the HOV lanes of Interstate 279. This led to a high-speed head-on collision that killed six people.
Opponents say that at the critical point when heavy traffic is about to become a traffic jam, the loss of the HOV lane from general use actually precipitates the traffic jam, making its usefulness a hollow victory. They also question how much ride-sharing they actually encourage, instead of merely advantaging those who would be sharing a vehicle anyway. HOV lanes added to existing highways also create more congestion at exits and interchanges because often no HOV-only exits are provided; the normal central position of the HOV lanes means that vehicles must transition across all lanes of traffic in a short distance to exit. Some places, like California, have started building special direct ramps so HOV lane users can change freeways or directly enter and exit the HOV lane without having to merge across all lanes.
Because HOV lanes seldom maximize vehicle throughput, a number of cities are considering converting under-utilized HOV lanes to high occupancy toll (HOT) lanes. This would permit single occupant vehicles to buy the right to use the HOV lanes for a toll, but total flow would be regulated (the price would be varied) to ensure total speeds on the HOV lane do not drop noticeably.
In some regions, buses are allowed to travel on the road shoulder when traffic becomes heavy, but it is often still illegal for cars (even highly-occupied ones) to take the shoulder to get around traffic jams.