Summary

High Occupancy Vehicle (HOV) lanes are designed to discourage single or low occupancy car use by providing priority to vehicles with more than a minimum number of occupants (usually two or three) and to buses. They encourage car sharing or public transport use, or both, 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 and are slowly but increasingly being implemented in Europe.
An HOV lane can be created by converting an existing multi-user road lane into an HOV lane (for all or part of the day), by adding an extra inside or outside lane to an existing road or by converting an existing bus way or bus lane into an HOV lane.
HOV lanes can be a part of motorway (as is common in the USA) or of a 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 may be defined by purely non physical means such as lane markings and special traffic signs. The hours and days of HOV lane operation can vary depending on the congested hours and function of the road stretch to which they are applied; for example during morning peak hours only, on weekdays or at all times.
Conclusions on the effects of HOV lanes vary from one study to other. They have been reported to reduce vehicle trips on HOV corridors by between 4 per cent and 30 per cent. It has also been reported that willingness to share cars and to use buses increases after the opening of an HOV lane. There is evidence that HOV lanes are more effective in these terms in cases where delays to general traffic due to congestion are greatest. There is, however, also a view that HOV lanes may be less effective in some cases than general purpose lanes.