HAZARDOUS MATERIALS: Substances that are harmful to human and environmental health and/or safety in relatively small quantities. Such materials include flammable, combustible, corrosive, reactive, and toxic substances as well as any other substance defined as "hazardous" by the state or federal government.

HERITAGE RESOURCES: See Cultural Resources.

HIGH OCCUPANCY TOLL (HOT) LANE: A High Occupancy Toll (HOT) lane is a toll enacted on vehicles accessing road facilities designated for the use of high occupancy vehicles (HOVs). Tolls for HOVs may be reduced or free. Implementation of HOT lanes should ensure adequate capacity to accommodate HOV traffic. High Occupancy Tolls area form of road pricing. Variable tolls are often used depending on congestion levels in the HOT lane.

HIGH OCCUPANCY VEHICLE (HOV) FACILITIES: Roadways or roadway elements that are restricted to use by carpools, vanpools, transit or other High Occupancy Vehicles during certain time periods. HOV facilities may consist of designated diamond lanes, or exclusive facilities separated from conventional traffic by barriers.

HIGH OCCUPANCY VEHICLE (HOV) LANE: Roadway or roadway elements that are restricted to use by carpools, vanpools, transit or other HOVs during certain time periods. HOV facilities may consist of designated diamond lanes or exclusive facilities separated from conventional traffic barriers. Policies on the minimum number of passengers using HOV lanes should be generally determined by travel characteristics of individual corridors, capacity and traffic volume of HOV lanes, and other operational considerations.

HIGHWAY CAPACITY: The maximum flow in vehicles per hour that can be reasonably expected on a particular segment of highway during a given time period under prevailing roadway, traffic, and control conditions.

HISTORIC OVERLAY DISTRICTS: A zoning tool used to regulate architecture in areas containing heritage resources to ensure compatibility with the resources. Site design, facades, and building materials must be reviewed and approved by an Architectural Review Board.

HOMESHARING: The shared occupancy of a house or apartment by two or more unrelated persons.

HOUSING TRUST FUND: A fund created by the Fairfax County Board of Supervisors to facilitate the creation of affordable housing within the County.

HOV: See High Occupancy Vehicle.

HYDROCARBON RUNOFF: The entrainment and transport by stormwater runoff of petroleum products, such as motor oil, originally deposited onto impervious surfaces by motor vehicles.

INDEPENDENT LIVING FACILITY: a residential development that is limited to occupancy by elderly persons and/or persons with disabilities. Such a facility shall provide: (a) dwelling units with complete kitchen facilities, (b) supportive services such as meals, personal emergency response systems, recreation and transportation services, and (c) design features, such as wider doorways and hallways, accessible-ready bathrooms and lower light switches.

INDUSTRIAL AREAS: The Land Classification System category for areas that are intended to provide suitable locations for industrially related uses. The category does not allow residential uses and generally limits future office uses to those which are ancillary to an area's industrial uses.

INFILL: Development on vacant or underused sites within an established development pattern.

INFRASTRUCTURE: The network of vital systems that comprises the improvements and publicly funded facilities added to the built environment to support new development and growth. Such systems include transportation, sanitary sewer, communications, stormwater, solid waste disposal, and water treatment and distribution.

INTENSITY: See Development Intensity.

INTERCOUNTY TRAVEL: Trips that originate in one county and have a destination in another county. For example, a trip with an origin in Fairfax County and destination in Prince William County, or vice versa, is regarded as intercounty travel.

INTRACOUNTY TRAVEL: Trips with both an origin and destination in the same county. For example, a trip that stays within Fairfax County is regarded as intracounty travel.

INVENTORY OF HISTORIC SITES: Properties included in the Inventory must meet certain eligibility criteria and are officially designated by the County's History Commission. Inclusion on the Inventory is primarily a way of publicly recognizing the significance of a property, although it does not legally protect a property from demolition or inappropriate change. Owners of properties included in the Inventory may meet with the County's Architectural Review Board (ARB) on a voluntary basis to review proposed changes to their properties.