Dahlgren 1994) in the US. They are designed to discourage single/low occupancy car use by providing additional priority to vehicles with more than a minimum number of occupants (usually two or three) and buses. They encourage car sharing and/or public transport use by allowing users to reduce their journey times relative to single-occupant vehicles, particularly when the general purpose lanes are congested. This in turn reduces the number of cars on the network and this reduction in the demand for road space can reduce overall congestion, fuel consumption and environmental impacts. They have become much more widespread in the last 10 years in the USA largely because legislation restricts the construction of mixed use roads in areas that fail US National Ambient Air Quality Standards.
In Britain, the National Travel Survey shows that between 1997 and 2002, average car occupancy rate dropped from 1.6 to 1.58, and single car occupancy is highest for commuting trips (85%). HOV lanes offer potential for making more efficient use of road space, especially at peak times, by increasing the attractiveness of public transport and retaining the flexibility and comfort of the car.

Two-wheel vehicles (motorcycles and/or bicycles) are usually permitted to use the HOV but this may vary between locations.
An HOV lane can be created in three different ways (ICARO, 1999):
• an existing general purpose lane can be converted into an HOV lane (temporarily or permanently);
• an additional inside or outside lane can be added ;
• an existing bus way/lane can be converted into an HOV lane.
Such schemes can therefore be considered as either preventing single-occupancy cars from using all of the available road-space or as providing additional road capacity for buses and HOVs.
HOV lanes can be a part of motorway (as in the USA) or major arterial road (as in Leeds, UK). They can be ‘tidal’ to help the traffic in the busiest direction (usually inbound towards city centres during the morning peak, outbound during evening peak). They can be permanent and separated by a physical barrier from the general purpose lanes or by purely non physical means (lane markings, special traffic signs, etc. (see illustrations below). The hours and days of HOV lane operation can vary depending on the congested hours and function of the road stretch that they are applied e.g. morning peak hours only, on weekdays or at all times.

In the USA there are now nearly 2,300 miles of HOV lanes

HOV lanes cover 60-mile of the I65/75 in Atlanta (picture above).

There are close to 100 HOV projects on freeways and in separate rights-of-way in 30 metropolitan areas in North America. A recent list of the HOV lanes and their details (e.g. length, number of lanes, time of operation) can be found at the Transportation Research Board: HOV Committee website (available at http://www.hovworld.com).

A Typical HOV lane in the USA

Changing the direction of a HOV lane

Courtesy of Transportation Research Board (http://www.hovworld.com/photos.html)
Terminology for HOV priority lanes generally also covers bus-only lanes (bus priority lanes). However, here we restrict attention to dedicated HOV lanes which both buses and ‘high’ occupancy private vehicles are allowed to use.
High Occupancy Toll (HOT) lanes are extensions of HOV lanes where ‘low’ occupancy vehicles (LOV) are allowed to use the lane if they pay a toll (or an increased toll) (Parsons Brinckerhoff, 2003).

Technology
Technology related to the HOV lanes other than the construction of these lanes is mainly related to its enforcement. It is generally necessary to allow HOVs to use both general-purpose and HOV lanes and to switch relatively-freely between them. If the general-purpose lanes are congested and the HOV relatively free-flowing, there will be temptation for LOV drivers to violate the HOV lane regulations. The success of the HOV lane operation therefore depends critically on appropriate enforcement. HOV lane enforcement involves determining eligibility to use an HOV facility by observing the number of vehicle occupants and penalising those who violate it.
Recently, automated occupant observation technologies have been under investigation. The ENTERPRISE program (see http://enterprise.prog.org) has been investigating ITS technologies for HOV lane enforcement. Possible options include outside observation technologies such as digital cameras, multi-camera video recorders, infrared radar machine vision and automatic vehicle identification. In the future, there may be some in-vehicle observation technologies available for HOV lane enforcement, but as yet no wholly-satisfactory solution has been found.
In the meantime, manual observation of vehicle occupancy will be required. Where HOV lane is physically separated from other lanes, random police controls at the entry or exit of the lane is usually relatively straightforward. Enforcement of HOV lanes with no physical barriers is more difficult. Alternatives include random checks by the police or other enforcers observing the use of HOV lanes and/or simply encouraging other drivers to report HOV lane violators. The latter approach obviously runs into difficulties if violators chose to simply deny an alleged violation.
Enforcement is crucial for gaining public support for HOV, but providing manual enforcement is expensive and takes the police away from their core work, while automated systems are still unproven. In Leeds, the cost of continuous police enforcement was a factor in the decision to restrict the lane to peak period operation.