Highway Functional Classification System

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Functional classification is the process by which streets and highways are grouped into classes according to the character of service they are intended to provide. Cities, towns, businesses, farms, homes, schools, recreation areas and other places generate or attract trips. These trips involve movement of vehicles through a network of roads. It becomes necessary to determine how travel movement can be channelized within a limited road network in a logical and efficient manner.

Functional classification defines the nature of this channelization process by defining the role that any particular road or street should play in serving the flow of trips through a road network. The heavy travel movements are directly served by major channels, and the lesser trips are channeled into somewhat indirect paths.

Possible translation from HFCS class to OSM highway tag
Interstates Interstates / Other Freeways & Expressways motorway
Other Principal Arterials trunk
Minor Arterial Roads Other Principal Arterials primary
Major Collector Roads Minor Arterial Streets secondary
Minor Collector Roads Collector Streets tertiary
Local Roads Local Streets unclassified(rural) / residential(urban)

History of the Functional Classification System

The concept of a functional classification system has been helpful to states and their DOT's for many years as a management tool in a variety of areas pertaining to highways. Federal, State and local governments use this tool to assign jurisdictional responsibility, allocate funds, and establish appropriate design standards for roadways. A functional classification for highways has been an important part of Federal-aid highway programs for many decades.

The National Highway Functional Classification study was mandated by Congress in the 1968 Federal-Aid Highway Act. Study procedures were developed to functionally classify all existing public roads and streets on the basis of their most logical use in serving current travel demands and land use. The study revealed that Federal-aid highway systems classification had become inconsistent with the function of roads and streets and adjustments in these systems were necessary.

The Federal-Aid Highway Act of 1973 required the use of an updated functional highway classification to modify Federal-aid highway systems by July 1, 1976. The functional classification study and Federal-aid highway systems realignment were completed by the State highway/transportation agencies in cooperation with local officials, and the results were approved by the Federal Highway Administration (FHWA).

After the 1976 federally mandated functional classification of highways was completed, States had routinely updated this functional classification to meet Federal-aid highway programs classification requirements. However, these adjustments resulted in the national functional classification of highways being no longer consistent among the States.

Through legislation of the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, the U. S. Department of Transportation recommended that a reclassification study be completed prior to designation of the National Highway System to provide an interconnected system of principal arterial routes that serve major population centers, intermodal transportation facilities, and major travel destinations.

In 1993, the functional reclassification was completed, and the National Highway System was established in November, 1995. The functional classification of 1993 has been routinely updated and is still of benefit today as an useful management tool.

The Concept of Functional Classification

Figure 1. The hierarchy of a functional classification system for a rural area.

The hierarchy of a functional classification system for a rural area is illustrated in Figure 1. The arterial highways provide direct service for cities and larger towns which generate and attract a large proportion of trips. The collectors serve small towns directly and connect them to the arterial network. Collectors also collect traffic from local roads, the bottom level of the classification system, which serve individual farms and other rural land usage.

Figure 2. The hierarchy of a functional classification system for an urban area.

A similar hierarchy of a classification system can be defined for urban areas as illustrated in Figure 2. However, because of the high intensity of land use and travel throughout an urban area, specific travel generation centers are more difficult to identify. Additional considerations, such as spacing, become more important in defining a logical and efficient network.

Traffic channelization provides access to property and various levels of travel mobility. Access is a fixed necessary requirement at both ends of any trip. Level of travel mobility refers to riding comfort, freedom from speed changes, and trip travel time.

Figure 3. The proportion of service provided by each functionally classified category

The proportion of service provided by each functionally classified category is illustrated in Figure 3. Arterial networks emphasize a high level of mobility for through traffic movements. Local facilities emphasize more on the land access function. Collectors offer a compromise between both functions of land access and mobility.

Organization of the Functional Classification System

The Functional Classification System is divided into rural and urban areas. The characteristics of these two areas are different in the types of land use, street and highway networks, and the nature of travel patterns. The urban areas are further divided into the small urban and urbanized areas.

Cities and towns are designated urban areas by the Bureau of the Census according to the population densities. An urban area with population between 5,000 and 49,999 becomes a small urban area. Any city or town with population less than 5,000 is a rural area. The urbanized areas are urban places with a population of 50,000 or more. An urbanized area might encompass several nearby small urban areas. For example, Oklahoma City urbanized area includes Edmond, Bethany, Moore, and many other smaller towns near or surrounded by Oklahoma City.

Functional Classification System For Rural Areas

Rural roads consist of those facilities that are outside of small urban and urbanized areas. They are classified into the following four major systems:

Classification Hierarchy For Rural Areas

  • Principal Arterials
  • Interstates
  • Other Principal Arterials
  • Minor Arterial Roads
  • Collector Roads
  • Major Collector Roads
  • Minor Collector Roads
  • Local Roads

Rural Principal Arterial System

The Rural Principal Arterial System consists of a connected rural network of continuous routes having the following characteristics:

  • Highways having high density of intrastate and interstate travel.
  • Highways that serve urbanized areas and a large majority of small urban areas. The rural principal arterial system may serve an urban area if the system either penetrates the urban boundary or comes within 10 miles.
  • Provide an integrated network without stub connections except where unusual geographic or traffic flow conditions dictate otherwise.

The Rural Principal Arterial System is classified into the following two subsystems:

  • Interstate System: Consists of all designated routes of the Interstate System.
  • Other Principal Arterial: Consists of all non-Interstate principal arterial highways.

Rural Minor Arterial System

The Rural Minor Arterial System should, in conjunction with the Rural Principal Arterial System, form a rural network having the following characteristics:

  • Link cities and larger towns and other traffic generators, such as major resort areas, that are capable of attracting travel over similarly long distances and form an integrated network providing interstate and intercounty service. A rural minor arterial system serves an urban area if the system either penetrates or comes within 2 miles of the urban boundary.
  • Be spaced at such intervals, consistent with population density, so that all developed areas of the State are within a reasonable distance of an arterial highway.
  • Provide service to roads with trip length and travel density greater than those predominantly served by rural collectors or local systems. Minor arterial routes have relatively high overall travel speeds and minimum interference to through traffic.

Rural Collector Road System

The Rural Collector Road System generally serves travel of primarily intracounty rather than statewide importance and constitute those routes on which (regardless of traffic volume) predominant travel distances are shorter than on arterial routes. More moderate speeds will be typical.

The characteristics of a rural collector system is subclassified according to the following criteria:

Major Collector Highways and Roads:

  • Provide service to any county seat not on an arterial route, to the larger towns not directly served by the higher systems, and to other traffic generators of equivalent intracounty importance, such as consolidated schools, shipping points, county parks, important mining and agricultural areas, etc.
  • Link these places with nearby larger towns or cities, or with routes of higher classification.
  • Serves the more important intracounty travel.

Minor Collector Roads:

  • Be spaced at intervals, consistent with population density, to collect traffic from local roads and bring all developed areas within a reasonable distance of a collector road.
  • Provides service to the remaining smaller communities.

Rural Local Roads System

The Rural Local Road System should have the following characteristics:

  • Serves primarily to provide access to adjacent land.
  • Provides service to travel over relatively short distances as compared to collectors or other higher systems. Local roads will constitute the rural mileage not classified as part of the principal arterial, minor arterial, or collector systems.

Functional System For Urban Areas

The four functional classifications for urban areas are principal arterials, minor arterial streets, collector streets, and local streets. The characteristics differences in rural and urban systems are the nature and intensity of development within the areas. The systems and their characteristics listed for urbanized areas are generally applicable to small urban areas. The basic difference is that, by nature of their size, many small urban areas will not generate internal travel warranting urban principal arterial service.

A principal arterial system for small urban areas will largely consist of extensions of rural arterial into and through the areas. The larger urban areas may have major activity centers which warrant principal arterial service in addition to that provided by extensions of rural arterials. The characteristics for the minor arterial street systems, collector street systems, and local street systems are similar for both urbanized areas and small urban areas.

Classification Hierarchy For Urban Areas

  • Principal Arterials
    • Interstates
    • Other Freeways & Expressways (Full or partial control of access)
    • Other Principal Arterials (No control of access)
  • Minor Arterial Streets
  • Collector Streets
  • Local Streets

Urban Principal Arterial System

In every urban environment there exists a system of streets and highways which can be identified as unusually significant to its surrounding area in terms of the nature and composition of travel it serves. These facilities in smaller urban areas (under 50,000) may be limited in number and extent. The importance could be primarily derived from the service provided to travel passing through the area. Principal arterials in larger urban areas could have the importance of service derived from traffic passing through the area, but equally or more important of service derived from major travel within these urbanized areas. The following are the main characteristics for streets and highways of the Urban Principal Arterial System.

  • Serves the major traffic movements within urbanized areas connecting central business districts, outlying residential areas, major intercity communities, and major suburban centers.
  • Serves a major portion of the trips entering and leaving the urban area, as well as the majority of the through traffic desiring to bypass the central city.
  • Provides continuity for all rural arterials which intercept the urban area.

The Urban Principal Arterial System is classified into the following subsystems:

  • Interstate
  • Freeways and Expressways (Full or partial control of access)
  • Other Principal Arterials (No controlled access)

The spacing of urban principal arterials will be closely related to the trip-end density characteristics of particular portions of the urban areas. No firm spacing rule can be established that will apply in all, or even most circumstances. The spacing of principal arterials in larger urban areas may vary from less than one mile in the highly developed central business areas to five miles or more in the sparsely developed locations.

The concept of service to abutting land for principal arterials should be subordinate to the provision of travel service with major traffic movements. Only facilities within the other principal arterial system are capable of providing any direct access to adjacent land, and such service should be purely incidental to the primary functional responsibility of this system.

Urban Minor Arterial Street System

The Minor Arterial Street System includes all arterials not classified as a principal and contains facilities that place more emphasis on land access than the higher system. This system should have the following characteristics:

  • Serves trips of moderate length at a somewhat lower level of travel mobility than principal arterials.
  • Provides access to geographic areas smaller than those served by the higher system.
  • Provides intracommunity continuity but does not penetrate identifiable neighborhoods.

The spacing of minor arterial streets may vary from 0.125 to 0.5 mile in the central business district to 2 - 3 miles in the suburban locations, but normally should not be more than 1 mile in fully developed areas.

Urban Collector Street System

The Urban Collector Street System differs from the arterial system in that facilities on the collector system may penetrate residential neighborhoods. The characteristics of the collector street system are as follow:

  • Collects traffic from local streets in residential neighborhoods and channels it into the arterial system.
  • Provides both land access service and traffic circulation within commercial areas, industrial areas, and residential neighborhoods.

Urban Local Street System

The Urban Local Street System offers the lowest level of mobility and the highest level of land access service. The characteristics for local street system are the following:

  • Comprises all facilities not on any of the higher systems.
  • Provides direct access to land and to higher road systems.
  • Through traffic usage is discouraged.

Data Sources

See also