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FHWA Functional Classification Guidelines

Section III - Suggested Procedures for Rural, Small Urban Area, and Urbanized Area Classification

Note: This publication was updated in 2013. This version is for archival purposes. For the current version, see http://www.fhwa.dot.gov/planning/processes/statewide/related/highway_functional_classifications/

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This section suggests procedures for classifying all roads and streets into functional systems for rural, small urban and urbanized areas, based on the most logical use of the existing facilities .1 to serve present travel. Separate procedures are presented for rural, small urban and urbanized areas. In addition, for each of these areas, procedures are given for a functional classification of existing conditions. Also, for each of those areas, procedures are given for a functional classification based on projected facilities and usage for some "future year."

While the basic concepts and functional criteria for the development of a "future year" functional classification plan are the same as those for a functional classification of existing facilities, it will differ in two basic respects: (1) It should be based on projected "future year" population, land use and travel; and (2) it will include, in addition to existing facilities, such projected totally new facilities as will be needed to serve "future year" land use and travel. Some of this new mileage will consist of new streets in expanding urban areas.

Beltways and bypasses in smaller cities will constitute another major category of new mileage. In addition, some new routes may be needed to serve planned and committed new recreational areas or new towns. A final category of additional, though in one sense not "new," facilities will be those representing relocation of existing facilities, in cases where adequate standards cannot- be provided on the original location, or where an existing routing is excessively circuitous.

In developing a "future year" classification, consideration should be given to the impact of foreseeable developments in other modes of transportation. On statewide systems, especially in heavily traveled intercity corridors, the influence of highspeed rail service and improved air service can be estimated through travel forecasts to the extent they are quantifiable. Such influences will probably have more impact on the needed capacity of highway facilities than on the actual system configuration.

"Future Year" - Functional Classification
When a functional classification is made based on a "future year," a projection of population should be made.

As was pointed out in Section II, the identification of population centers is essential in the functional classification concept. When a "future year" functional classification is made, population estimates for that "future year" should be prepared for all areas that are expected to be urban as well as for the remaining rural subareas.

Each populated place presently containing less than 5,000 persons and not included within the delimited boundary of a "future year" urbanized area, should be examined to determine whether its anticipated population growth to the "future year" will result in its classification as a small urban area. In addition, certain presently rural areas (i.e., suburban development, new towns, etc.) should be examined to determine those which will qualify as small urban areas due to expected population increases by the "future year."

The base for a "future year" population should be the most recent Decennial Census. As applicable, the total State regional and national "future year" populations should be given consideration when estimating populations of the individual urbanized and small urban areas in order that the estimates will be reasonable and consistent. Consequently, in making "future year" urban estimates, it will be necessary to develop them coincidently with and in relation to the total "future year" State population projections and the projections for the remaining rural population (including those places from 2, 500 to 4,999 population).

A considerable amount of population data is available in the States through the urban transportation studies, from previous functional classification studies (see page I-1) , and from agencies preparing current population estimates for the various States.

Because of the variety of kinds of population forecasts and sources of forecasting advice and assistance that are available to the States, no single forecasting procedure is suggested in this manual. Of foremost importance in any procedure is the maintenance of a sound overall perspective. Specifically, the aggregate of individual place projections must stand the test of reasonableness in terms implied overall trends for urbanized areas, for small urban areas by size group, and for rural area density.

To assure reasonable distribution of total projected population by the above categories an iterative approach with feedback tests is necessary, particularly, in some States, when a very large proportion of the total population growth will occur in urbanized areas. Proportionally small variances in forecasts for these places can have a disproportionate effect on residual values applicable to small urban places and rural areas. Hence a stepdown residual forecasting procedure without feedback should be avoided.

CLASSIFICATION PROCEDURES FOR RURAL SYSTEMS

Rural classification procedures apply to those areas outside of urbanized or small urban area boundaries, although many rural routes particularly arterials, continue into or through the latter areas.

Identifying and Ranking Population Centers and Other Travel Generators
The procedure for rural functional classification, as outlined in this subsection, initially involves connecting traffic generators in such a manner as to logically channelize the trips on the road network. Since most trips begin or end in a city of town, population centers are the primary traffic generators considered. However, since travel is also generated by recreation areas, such as National parks, ski resorts, lakes, and beaches, that have little resident population, instructions are included here for comparing the importance of these areas to that of a city or town.

The population of a place generally reflects its capacity for generating and attracting travel. Socio-economic factors, such as trade, employment, etc., may also indicate the importance of a place in relation to intercity travel. Urban areas of similar population and economic activity (and consequently travel generation and attraction) should be identified and service provided to them by routes of the same statewide functional system.

Ranking of population centers, usually on the basis of population is an initial step in the classification process. Available socio-economic data (e.g., sales tax receipts, retail trade, employment, etc.) may be used along with population in this ranking if the State feels that such factors are significant for the area under study. Each urban area should be treated as one center, even if several jurisdictional units are involved and even if part of the population is in an adjoining State.

Since this ranking process is one of the means of determining the population centers for which service by a particular functional system is to be provided, all places thought qualified for service by the major collector road or any higher system should be' ranked.

Major travel generators other than cities, such as recreation areas (National and State parks, State fairgrounds, ski resorts, lakes, beaches, etc.) and military installations should be treated separately during the ranking process because of their unique, predominant land activity. Usual trip generation yardsticks, such as population, employment, and related factors which measure the socio-economic status of the area and its population, are not applicable to such generators because of their atypical travel generation potential. For example, National parks and State fairgrounds contain little or no resident population and, in general, contain no commercial or industrial activity other than facilities to serve tourists. Hence, these centers require that other data be employed during the ranking process.

For purposes of functional classification, the annual number of visitors to such a recreation area can be equated to an urban area's population as shown in Figure III-1. The recreation area can then be grouped with population centers of similar trip generation potential, and service provided by the same functional system.

Figure III-1. Visitation VS. Equivalent Population For Ranking Recreation Generators. Visitation vs. Equivalent Population for Ranking Recreation Generators. The intent of this figure is to show that sites that attract large numbers of visitors should be considered when you lay out your arterial road system. Sites with large numbers of visitors can be equated to urban areas of a particular size by observing the relationship depicted in the graph. The graph has the vertical axis labeled Equivalent Urban Area Population in thousands with a range from 1,000 to 1,000,000. The horizontal axis is labeled Annual Number of Visitors in hundreds of thousands, ranging from 100,000 to 10,000,000. The relationship pictured is a straight line, which shows that 300,000 visitors is the same as an urban area population of 1,000; 600,000 visitors is the same as an urban area of 4,000; 1,000,000 visitors is the same as an area of 10,000; 2,000,000 visitors is the same as an area of 30,000 and 10,000,000 visitors is the same as an area of 400,000.

Figure III-1 - Visitation VS. Equivalent Population For Ranking Recreation Generators

Where several recreation areas are located close together and can be served by only one possible route, such as on a coastal peninsula or in a mountainous area, the equivalent populations may be combined in ranking the area.

Visitation data for recreation areas administered by the State and Federal Governments should be available from the Bureau of Outdoor Recreation liaison officer in each State.

The importance of recreation and other generators can be inferred from traffic flow data if there are no other data available for ranking purposes.

Classification of Rural Systems
As stated earlier, the procedure for rural functional system classification initially involves connecting traffic generators in such a manner as to logically channelize the trips on the road networks. The preceding discussion explains procedures for ranking population and other centers of traffic generation. These procedures do not eliminate judgment from the classification process, but when used as a guide they do help to apply judgment in a sound and orderly fashion.

Rural principal and minor arterial systems
The procedures for functional classification of rural roads into the principal arterial and minor arterial systems are described in the following enumerated steps:

  1. One of the initial steps in the classification of rural routes is the preparation of road network maps. Maximum use should be made of existing maps although reference to administrative or jurisdictional systems should not be considered in the classification process.

  2. Rank travel generators as described in the immediately preceding pages. Plot generators graphically, in order to ranking, and divide into groups, with centers of similar rank in each grouping, as illustrated in Figure 111-2. While no hard and fast rules apply, six to eight groupings will usually be typical. Too many are better than too few, particularly toward the lower end of the scale. This ranking and grouping will aid in determining which centers qualify for minor arterial service or major collector service, and which will be adequately served by minor collector roads.

  3. Identify pertinent travel generators in adjoining States. Judgement should be used in selecting the centers to be included. Larger out-of-State generators have traffic attraction relationships over a considerable distance while smaller out-of-State generators may be of influence only when close to the State boundary. Fit these selected out-of-State generators into the appropriate size group determined for the in-State generators in Step 2 above.

Figure III-2. Visitation VS. Equivalent Population For Ranking Recreation Generators. Graphic Ranking and Grouping of Travel Generators for a typical state. When you list the major travel generators in your state, usually urban areas, you should assign them to different size groupings with the largest generators assigned to group 1 and the smaller size areas assigned in descending order to groups 2 through 7. The graph shows the relationship between population size and the groups. The vertical axis is labeled population and the range is 0 to 100,000. The horizontal axis is label rank and the range is 0-50. Generators above 75,000 population are assigned to group 1, generators from about 15,000 to 75,000 are assigned to group 2; generators from 4,000 to 15,000 are assigned to group 3, generators from 2,200 to 4,000 are assigned to group 4, generators from 1,500 to 2,200 are assigned to group 5, generators from 750 to 1,500 are assigned to group 6 and generators from 250 to 750 are assigned to group 7.

Figure III-3 - Visitation VS. Equivalent Population For Ranking Recreation Generators

  1. Develop a map symbol (for example, a simple open or lightly shaded circle) for each size group of travel generators, with the size of the map symbol indicating the population range of centers in the group. Plot the generators on a statewide map. A tracing overlay superimposed on the statewide road map is recommended. The few pertinent out-of-State generators which may fall outside the State map can be dealt with by plotting them on a regional map. once the appropriate routings to the out-of-State generators have been selected, they can be shown on the statewide map by placing arrows at the State line.

  2. Delineate urbanized area boundaries on the statewide map as accurately as practicable. (Subsequent accurate mileage determinations will probably require reference to large-scale maps, particularly when measuring mileages within urban limits.)

  3. Delineate all presently designated routes of the Interstate highway system.

  4. Select the remaining rural principal arterial routes and, following that, the rural minor arterial routes, in a general sequence that will "work down from the top" to reflect a gradation of the following route characteristics, considered in combination: (a) Size of travel generators connected; (b) predominant travel distances served; and (c) size of tributary area or "travel shed" served. The term "in sequence" does not mean an exact numerical ranking of routes since in many cases several routes may be deemed nearly equal in the above characteristics.

Figure III-3. Plot of Cumulative Road Mileage Versus Cumulative Vehicle Miles Served. Plot of Cumulative Road Mileage versus Cumulative Vehicle Miles Served for a typical state. This graph shows that the higher functionally classed roads serve most of the traffic although they represent only a small percentage of the roads. The vertical axis is labeled Cumulative percentage of rural vehicle miles and it ranges from 0 to 100%. The horizontal axis is labeled Cumulative percentage of rural road miles. The relationship rises steeply and then tapers off. The critical points on the graph show that 8% of the arterial roads serve 73% of the travel, collectors serve 19% of the travel on 25% of the roads and local roads serve only 8% of the travel on 67% of the roads.

Figure III-3 - Plot of Cumulative Road Mileage Versus Cumulative Vehicle Miles Served

  1. Add such other routes to the minor arterial system as are required by the defined system characteristics. Such routes will include:
    1. (Service to corridor movements with trip lengths and volumes equivalent to those of routes already added, as determined from traffic flow maps.
    2. Service to all areas of the State, with spacing of routes at reasonably consistent intervals, as tempered by consideration of population density.
    3. Such additions as are clearly needed for adequate statewide continuity (but only where significant travel patterns serve to justify them).
  2. Inclusion in the system of additional alternative routes is a problem that will occasionally arise. In most cases a single connection between two centers is all that is needed. Some instances where alternative routes may have to be considered are:
    1. Where two apparently alternative routes are separated by geographic barriers and each is needed for minor arterial service to some qualified intermediate center or for connection to another intersected minor arterial route.
    2. Where one major facility is a parkway from which commercial vehicles are excluded.
    3. Where the total traffic volume cannot practicably be handled by one facility.
    4. Where one facility is a toll road.

"future year" Classifications:
Studies conducted over the years have indicated a large degree of stability in the routes and corridor locations of arterial systems. To a considerable extent, centers of the lower size range of places served by these systems (especially minor arterial) are not undergoing great or rapid change. Furthermore, considering mere growth, per se; if all centers were growing in proportion, without causing significant shifts in travel linkages, such growth would not affect the functional relationships in the road network. There will, however, be instances where smaller cities and towns, due to unique circumstances of location or activity, will be anticipated to undergo substantial growth. The same will apply, probably in greater degree, to other travel generators, especially recreation centers. These rapidly expanding generators will be of principal interest in reviewing the updated ranking of generators.

Generators other than population centers should be involved in the ranking of generators. Both in regard to population projections and in projecting these other generators, statewide and regional development agencies should be contacted to obtain information on development trends, available socio-economic forecasts, and statewide and regional development plans.

Visitation forecasts for important recreation centers should be obtained, or made if not available. Projections of visits should be reviewed to assure that individual forecasts are realistic in terms of use potential and that projected statewide totals reflect a growth rate consistent with overall travel growth. Figure III-1, page 111-5 may be used to obtain equivalent population to use in the ranking process.

Rural collector system
The step-by-step procedure just described for laying out the rural principal arterial and minor arterial systems can be extended in a qualitative sense, to the development of the rural collector system. However, precise quantitative data as to size of traffic generators and amount of traffic movement are usually not available to the same degree at the collector level. Also, population density and distribution and basic road patterns vary widely at this level. Accordingly, the procedure as described here is somewhat more generalized than that described for the higher systems. In any case, it should be borne in mind that what is being laid out is the backbone network of traffic circulation at the county or local level.

Before selecting any routes for the rural collector system a preliminary visual and mental assessment of the entire local picture should be made, considering the following:

  1. Location of population centers (including county seats) not already served by the higher systems.

  2. Location of important local traffic generators other than population centers: consolidated schools, shipping points, county parks, etc. Aerial photographs, where available, should prove helpful in locating these local traffic generators.

  3. Location of any heavier-than-average corridor movements within the county, from traffic flow data.

  4. Location of existing freeway interchanges or important river crossings that may be key location controls with regard to the collector system.

  5. Rural population and land-use distribution within the county as regards uniform or nonuniform density of development.

Selection of major collector routes. --In many instances, selection of a few major collector routes can be made and shown on the statewide map which has been used to delineate the arterial systems. This is a practical matter of working with whatever map offers the most convenient scale. Completion of the collector classification, however, should be done on maps of county scale, preferably those of the county highway planning series. A mosaic of maps of the county being classified and the bordering counties will be helpful in determining the function of routes crossing the county line. The designated principal arterial and minor arterial systems and any collector routes already designated on the statewide map should be transferred to the county map before any additional routes are selected. The major collector routes should then be selected to accomplish the following:

  1. Connect the county seats and the larger population centers not served by the higher systems with such systems and/or directly with nearby larger population centers served by those higher systems.

  2. Link the more important local traffic generators with nearby population centers or with this or a higher system.

  3. Serve corridor movements with traffic volumes and trip lengths comparable to those of major collector routes already selected.

Selection of minor collector routes. --The routes selected up to this point serve to connect population centers and other traffic generators of like magnitude. However, there will be many areas with clustered residents at considerable distance from the previously selected systems. Within reasonable economic limits, minor collector or "spacer" routes should be designated to serve these areas, interconnect the small communities, and link the locally important traffic generators with their rural hinterland.

These "spacer" routes should be selected so as to provide approximately equal distance between arterial or collector routes for equal rural population densities so that equitable service is provided to all rural areas of the State. The approximate population density within each area bounded by major collector or arterial routes can be determined, either from census data or by an approximate house count from the county highway map, and the existing spacing of routes already selected can be measured. Areas with poor service can then be identified by comparing those data with a table of desirable collector spacing (miles between routes) versus population density (people per square mile) and additional routes selected and added to the collector system where necessary.

Future year classification. --In most counties there should be a substantial degree of stability over time in the extent and location of rural collector routes. There will, of course, be changes brought about by (a) change urban-in-fact boundaries, (b) reclassification of arterials superseded by relocations; even in counties where the rural environment remains little changed, and, (c) reclassification of roads presently functioning as collectors to local classification due to the normal diversion and increased channelization of traffic on to one facility following a highway improvement.

Probable changes in land use which would significantly affect the classification plan should be forecast wherever possible. Such changes are most predictable where substantial recreation developments are being planned or where other changes in basic economic activity can be firmly projected, including some assurance as to probable activity sites. Plans and forecasts of State and local agencies should be sought out where available. It is not suggested here, however, that all local plans be uncritically accepted. They should be compared with overall State forecasts for reasonableness.

Local rural roads
The remaining rural mileage not otherwise classified as principal arterial, minor arterial, or collector should be assigned to the rural local road system.

For future year classifications there will generally be a reduction in rural local mileage brought about by changed urban boundaries. There may be some growth of rural local mileage, particularly for projected recreation, industrial and rural residential developments.

CLASSIFICATION PROCEDURES FOR SMALL URBAN AREAS

This subsection includes the procedures for developing functionally classified street and highway systems in small urban areas. The systems so developed should be consistent with the system characteristics discussed in Section II.

Determine and map the urban area boundary
The boundary delimiting the area that is urban-in-fact, should be plotted on an existing map of the small urban area. Existing land-use maps or recent aerial photographs may be used to help in locating this boundary. Where neither of these are available, the division line between urban and rural development can be determined through aerial or ground reconnaissance; or officials of the town under study may help to locate this line from their knowledge of local development.

Prepare road network map
The street and highway network should be updated on the map used in selecting the urban boundary by adding any facilities open to traffic that are not shown on the original map. New routes can be sketched on the map in their approximate location.

Identify and map land service characteristics
Major traffic generators, land use patterns, and the points at which rural arterial and collector routes intercept the urban boundary should be identified and shown on the map of the area. Recent aerial photographs should prove very useful in identifying the major traffic generators and land use patterns.

Classify the highway and street network
Classify the highway and street network in accordance with the system characteristics discussed in Section II, and in relation to the land service characteristics described above. In accordance with logical system continuity considerations, select first the principal arterial system, followed by minor arterials, and finally collectors.

As a first step in this process, the Interstate System should be identified on the map. Next, any sections of other freeways or expressways should be delineated. Additional routes should then be selected to provide continuity through the urban area for the routes already identified and for all other rural principal and minor arterials intercepting the urban boundary. In urban areas under 25,000 population, the principal arterial system will probably consist wholly of routes such as the ones selected above. In those small urban areas over 25,000 population, however, there may exist urban activity centers of regional importance. Where these centers do exist, routes should be added to the principal arterial system so that adequate service is provided.

Next, minor arterial streets should be designated to serve the remaining urban activity centers and to provide adequate areawide circulation. The reasonableness of route spacing should be considered, using the quidelines shown below in Table III-1.

Table III-1--Arterial spacing guidelines
Area type Arterial spacing
Central business district 1/8-1/2 mile
Urban (central city except CBD) 1/2-1 mile
Suburban 1-2 miles

Finally, the collector streets should be selected, based on the systems characteristics discussed in Section II, and delineated on the map of the urban area. Remaining streets, of course, will form the local street system.

"Future Year" Classifications. --A functional classification for "future year" system plans in small urban areas can be developed as follows:

  1. Develop, in general concept, the pattern of future land uses in presently undeveloped areas within and around the city. Assumptions must be made (realistically) regarding major new commercial, industrial, institutional, and recreational developments as well as residential development. In the absence of a "future year" land use plan, guidance must come from the pattern of land use in the present urban area (particularly from recent growth, if any),, for local knowledge of any development proposals, from the pattern of existing road network, from the effect, of other transportation facilities, and from an examination of the terrain conditions in the area.

  2. considering the above and the urban boundary criteria discussed on page 11-7, delimit the "future year" urban area boundary.

  3. Using the latest available functional classification as a base, delineate the principal arterial and minor arterial street networks within the future year urban area boundary. Included in these networks will be projected new facilities based on the land use plan or the assumption developed in (a) above.

  4. Evaluate (for reasonableness) the extent of the projected mileage of new facilities developed in (c). Miles of arterials per square mile of area should be comparable to the rate in areas presently developed to a similar land use intensity. This miles-per-square-mile rate for facilities in the area of future urbanization should logically not be higher than the corresponding rate for the present urban area, since the latter includes the densely developed areas of the city.

  5. Projecting proposed locations for future collector and local streets in presently undeveloped areas may, in many cases, be impracticable. However, statistical estimates of future collector and local street mileage may be desired, particularly as a basic for projecting maintenance requirements. Statistical indices, such as a street-miles-per-square-mile rate, may be developed, based on existing developments at dwelling unit or population densities similar to that projected for the new area.

  6. Evaluate the adequacy of the overall classification plan to serve anticipated future year travel. The following questions, among others, should be considered: Does the pattern of principal arterials (if any) plus minor arterial streets provide adequate continuity for citywide movement? Can anticipated future year capacity requirements be met within developable rights-of-way of the designated network or should additional arterials (oneway couplets, for example) be designated? Would such added arterials, in regard to their impact on the immediate environment, be representative of realistic proposals that might be implemented to satisfy local demand? Has the distinction between arterial and collector streets been properly and consistently defined?

  7. Develop the further subclassifications within the principal arterial street classes required to provide connecting links for the rural principal arterial and minor arterial systems as described on page 11-15.

Introduction
This subsection of the manual presents a procedure which can be used to develop functionally classified street and highway systems in urbanized areas. No such procedure can be used mechanically or without judgment. Rather, it is intended to serve as a guide, and if proper application is made of the definitions and criteria, the resultant systems will be fully appropriate for this nationwide study and should provide an excellent base for local transportation planning.

It should be mentioned at the outset that the procedures presented in this section are suggested as a logical approach to urban functional classification. They are designed to conform with the needs and capabilities of most of the urbanized areas. For those areas in which all of the procedures outlined here cannot be followed, the suggested methods may still be adhered to as closely as available data permit.

Listed below are the basic steps which comprise the suggested procedure for functional classification in urbanized areas (each step is discussed in the following text):

  1. Determine and map the urbanized area boundary.

  2. Map the road network.

  3. Perform a preliminary classification of the total arterial system.

  4. Classify the final arterial system.

  5. Classify the principal and minor arterial street systems.

  6. Substratify the principal arterial system.

  7. Classify collector and local streets.

Classification Procedures for Urbanized Areas

  1. Determine and map the urbanized area boundary
    The definition of urban area is given on page 11-7. Federalaid urban area boundaries are established in accordance with Volume 4, Chapter 6, Section 3 of the Federal-Aid Highway Program Manual.

  2. Map the road network
    A base map should be prepared containing the street and highway network within the urbanized area. In most urbanized areas, preparation of such a map will simply involve updating existing maps.

  3. Perform preliminary classification of the total arterial system
    The preliminary classification is directed toward establishing a tentative division between arterials and all other streets and highways, based upon all available criteria. Where the choice between arterial and collector is borderline or unclear, the facility should be included in the preliminary arterial system. Resolution will come with more detailed analysis in the final arterial system classification when additional criteria may be applied.

    Functional system criteria are related to trips served, areas served, and characteristics of the facilities themselves. Within this basic framework, specific measures can be identified as being particularly applicable in assigning facilities to predefined functional classes. For urban functional classification, the criteria measures deemed most useful include service to urban activity centers, system continuity, land use considerations, route spacing, trip length, traffic volume, and control of access. Naturally, none of these can be applied independently, or to the exclusion of all others, in developing functional systems. It is hoped that as many of these as are feasible will be considered in arriving at a logical functional classification. The application of these criteria in classifying a preliminary arterial system is described below.

    1. Service to urban activity centers
      The greater the importance of an urban activity center, in terms of the nature and quantity of travel generated, the wider is its range of trip attraction and, therefore, the greater its need to be served by a higher type system. Some urban activity centers may be evaluated for relative importance by quantitative measures of size and intensity of use, such as number of employees, trip-end density, and the like. In determining the hierarchy of trip generation centers, it may be helpful to consider them in groups arranged according to such measures. These can be plotted from high to low, in the manner shown in Figure 111-4. Such an analysis may be useful in identifying the trip generators that should be served by each functional system. Typically, there are comparatively few very large generators in an urbanized area and these should be served by the principal arterial system.

      Where urban activity centers of social and economic importance to the area cannot be weighed quantitatively, they should be identified, subjectively ranked, and appropriately served by the principal or minor arterial system as warranted. Subjective comparison of the relative importance of these centers to those of the first type may be helpful.

      Centers appropriately served by arterials should generally include traffic generators of regional or community importance. These consist of the business districts of the central city as well as those of satellite communities, shopping centers, recreational facilities which serve larger than purely local areas, transportation terminals, industrial centers, large high-density residential developments, and the like. These travel generators may be considered to be served by arterials if such a facility passes within one-quarter to one mile of the limits of the activity center, depending upon the type of arterial and the size of the generator. All trip generators which warrant arterial service should be located on a suitable map or overlay, identified according to relative importance.

Figure III-4. Distribution of Development Intensities for a Typical Urbanized Area. Distribution of Development Intensities for a Typical Urbanized Area. The vertical axis is labeled Development Intensity (trip end density) and there are no numers depicted on the axis. The horizontal axis is labeled Rank and there are no numbers depicted either. The relationship drops rapidly and then it flattens out. The concept is that if you listed the travel generators from highest to lowest on the horizontal axis you would see that the largest generators generate large numbers of trip ends. After the first highest generators are plotted you can see that the next group generate much fewer trips. The idea is that a few generators generate the majority of the trips.

Figure III-4. Distribution of Development Intensities For A Typical Urbanized Area

  1. System Continuity
    The arterial system should be completely integrated, with stub ends occurring only at the urban area boundary (in which case they connect with a rural arterial or a rural collector) or in areas having unusual topographic features, such as sea coasts.

    In rare instances, system continuity should not be an absolute constraint for the functional classification of systems. Exceptions could be permitted where long-distance trips end at major centers, such as airports.

  2. Land-use considerations
    Land use is a primary consideration in functional classification, for the mosaic of existing land use largely governs overall travel patterns, travel density, and street spacing.

    The transportation system is a major structural element of the community. It serves as a circulatory system providing travel mobility, but it serves equally as a skeletal system providing a relatively permanent framework which delineates and influences the pattern of land development, and within which residential neighborhoods and other land uses may develop and function. The preservation of neighborhoods, the stabilization of desirable land uses, and the encouragement of orderly development are among the basic considerations in the development of functional street systems.

    The concept of streets as a land use is also important in functional classification. In the same manner that industrial activities usually make undesirable neighbors for residential districts, but make suitable neighbors for railroads, so must streets and traffic be viewed in terms of their impact upon as well as service to adjacent land uses. The classification of streets into functional types recognizes this and encompasses, at one extreme, local streets which furnish access to abutting land and discourage through-traffic movement, and at the other extreme, arterials which furnish a primary service to through travel and avoid penetrating identifiable neighborhoods where possible. Establishment of functional street systems and unification of these systems into a balanced network are basic to comprehensive urban planning and must be concurrently accomplished as an integral component of urban planning procedures.

    Using suitable overlays on the base transportation network, maps should be prepared which identify all sizeable areas of similar land-use characteristics, such as industrial, commercial, institutional, open space, or residential. Maps such as this are readily available in most urbanized areas in a-form requiring little or no additional work.

  3. Spacing between routes
    The geometric configuration of highway and street systems must be related to the spatial distribution of the activities to be served and to the density of traffic generated. Generally, the more intense the development, the closer the spacing required. In the less dense suburban portions of an urbanized area, neighborhoods tend to be larger than in the more dense central cities. These less dense areas will not require the same close spacing of facilities to serve traffic as the areas closer to the central business district (CBD).

    Based upon these considerations Table 111-2 presents a general indication of desirable arterial spacing according to type of area. In addition, Figure 111-5 provides a measure of theoretical arterial spacing required to serve travel to varying intensities. It is recognized that neither the spacing guidelines included in the table nor the theoretical spacing reflected by the curves in Figure 111-5 will apply universally to the spacing of existing arterials. However, they may prove particularly useful in borderline cases where other criteria cannot fully indicate the appropriate functional class of a particular facility.

Table 111-2 -- Arterial spacing guidelines
Area type Arterial spacing
Central business district 1/8-1/2 mile
Urban (central city except CBD) 1/2-1 mile
Suburban 1-2 miles
Lowest density development 2-3 miles

Figure III-5. Minimum Theoretical Arterial Spacing Required to Accommodate Arterial Travel Demand at Route Capacity. Minimum Theoretical Arterial Spacing Required to Accommodate Arterial Travel Demand at Route capacity. This graph shows the relationship between travel per square mile and the spacing of arterial roads. The vertical axis is labeled Spacing in miles and ranges from 1/10 to 2 miles. The horizontal axis is labeled Daily vehicle miles of travel per square mile and ranges from 0 to 300,000. There are three lines shown on the graph, the first is label 2 lane arterials and it begins at a spacing of 2 miles and it declines rapidly as the travel density increases. When you have a travel density of 50,00 you need 2 lane arterials every 1/4 of a mile. At 25,000 VMT, you need them every 1/2 mile and at 10,000 VMT, they can be spaced every mile. The next line is labeled 4 lane arterials and it decinens as travel increase but not as rapidly as it did for 2 lane arterial roads. At 50,000 VMT, you need 4 lane arterials spaced about 1 mile. At 100,000 VMT, they should be spaced every 1/3 of a mile. At 150,000 VMT, they should be spaced every 1/4 mile. The last line is labeled 6 lane arterials. At 50,000 VMT they should be spaced every 2 miles, at 100,000 VMT every 3/4 miles, at 200,000 VMT every 1/3 mile.

Figure III-5. - Visitation VS. Minimum Theoretical Arterial Spacing Required to Accommodate Arterial Travel Demand at Route Capacity (Illustration scanned)

  1. Average trip length
    A basic assumption in assigning facilities to logical functional groupings is that higher order systems should generally serve the longest trips. Figure 111-6 illustrates a characteristic high-to-low ordering of average trip lengths on segments of a highway network in a large urban area. - Only comparatively few miles of urban streets and highways serve trips of any great length; a somewhat greater mileage serves trips of moderate length; and a substantial mileage serves comparatively short trips. The approximate break points between these triplength groupings can suggest possible ranges of average trip length for each of the functional system.

    A quantitative measure of average trip length on a facility can be obtained if desired via the traffic assignment process. However, it is also possible to apply this criterion in a generalized way without the benefit of quantitative measurements. This requires a knowledge of the nature of travel served by individual roads. Facilities which serve relatively long trips (including trips passing through the urban area, trips between the suburbs and central city, trips between outlying communities, and long trips occurring within the central city) are likely to be functioning as arterials and should be considered for inclusion in the preliminary arterial system.

    An exception in application of the average trip length criterion lies in the existence of outlying minor routes which, by virtue of their distance from the metropolitan center, may carry an unusually high proportion of long trips; indeed, longer average trip lengths than on some principal arterials located closer to the center of the metropolitan area. Consequently, it is necessary to consider trip length within the basic framework of other criteria that reflect the other characteristics of a facility as well as the type of area the facility is in.

Figure III-6. Average Trip Lengths on Segments of an Urban Street System. Average Trip Lengths on Segments of an Urban Street System for a typical area about 1,000,000 population. This graph shows the relationship between average trip lengths and total travel. The idea is that although there are relatively few long trips, they are responsible for a large share of total travel. The vertical axis is labeled Average trip length and it ranges fro 0-45 miles. The horizontal axis is labeled cumulative mileage on the arterial system and it ranges from 0 to 800 miles. The relationship begins at an average trip length of 45 miles and drops rapidly at first, then it flattens out. Average trip lengths of about 15 miles account for about 100 cumulative miles of arterial roads. Average trip lengths of 10 miles account for 200 cumulative miles and average trip lengths of 5 miles account for 500 cumulative miles.

Figure III-6. - Average Trip Lengths on Segments of an Urban Street System

  1. Traffic volume
    In functional classification, the routes with the highest traffic volumes are likely to be included in the highest type systems, although this is by no means a firm rule. To assist in developing specific volume criteria for an individual urban area, it is suggested that a list of volumes on individual route segments be plotted (from high to low) against the mileage of routes included as illustrated by Figure 111-7. Notice that there are usually relatively few miles of the system that carry high volumes and a modest mileage carrying moderate volumes, but that most mileage comprises low-volume routes.

    Most high-volume streets and highways in an urban area function as arterials. But there are exceptions, notable in intensely developed areas where high-volume facilities function as collectors, serving traffic movements between local streets and arterials, or providing a high degree of direct access service to abutting property. For example, some roads which border on large traffic generators may carry proportionately high volumes of traffic while functioning as collectors.

    To use the volume criterion as an aid in establishing a preliminary arterial system, it is desirable to have traffic volume data on all segments that probably will be classified as arterials and on all or most facilities which will eventually comprise the "upper" portion of the next lower functional class of roads. This is necessary for determining the approximate volume range in which the break between arterials and collectors occurs (considering the exceptions noted above) , as exemplified by the curve in Figure 111-7. Traffic volume flow raps as well as a rank order distribution of road segments based upon volume can also assist in the analysis.

    It is not intended that traffic counts be made specifically for this analysis. Rather, it is hoped that extensive use will be made of the most recent data already available.

Figure III-7. The Relationship Between Travel Density and Cumulative Mileage on the Arterial System. The Relationship Between Travel Density and Cumulative Mileage on the Arterial System in a typical large urbanized area. This graph shows the relationship between VMT per mile and cumulative arterial mileage. The vertical axis is labeled VMT per mile in thousands with a range of 0 to over 100,000. The horizontal axis is labeled cumulative mileage on the arterial system with a range from 0 to over 800. The line drops rapidly and then flattens out. VMT per mile over 50,000 accounts for about 75 arterial miles. VMT per mile over 20,000 accounts for about 100 arterial miles. VMT per mile over 10,000 accounts for about 300 arterial miles and VMT per mile over 5,000 accounts for about 800 arterial miles.

Figure III-7. - The Relationship Between Travel Density and Cumulative Mileage on the Arterial System (Illustration scanned)

  1. Control of access
    Control of access is perhaps the easiest criterion to apply, since facilities with full or partial control of access will almost always be in the arterial class. It may therefore be advantageous to delineate these facilities at the very outset, thereby providing for a convenient starting point in defining a preliminary system of arterials.

  2. Vehicle-miles of travel and mileage
    The extent of vehicle-miles of travel and system mileage to be included in the preliminary arterial system classification should be on the high side of the values entered in Table 11-3. This will be the natural outcome of including in this system all facilities about which serious question remains as to whether they are arterials or collectors. It is logical to include such facilities initially in order that they may be subjected to the more stringent analyses described in step D.

Figure III-8. Plot of Cumulative Urban Street Mileage Versus Cumulative Miles ServedPlot of Cumulative Urban Street Mileage versus Cumulative Vehicle Miles Served for a typical urbanized area. The concept depicted is that the higher functional classes carry the bulk of traffic even though they represent a small percentage of the roads. The vertical axis is labeled Cumulative percentage of urban vehicle miles and it ranges fro 0 to 100%. The horizontal axis is labeled Cumulative percentage of urban street miles and it ranges fro 0 to 1000%. The curve rises rapidly and then tapers off. It shows that principal arterials carry 48% of the traffic on 8% of the street miles. Minor arterials carry 26% of the traffic on 16% of the street miles. Collectors carry 5% of the traffic on 6% of the streets and locals carry the remaining 21% of the traffic on 71% of the streets.

Figure III-8. - Plot of Cumulative Urban Street Mileage Versus Cumulative Vehicle Miles Served (Illustration scanned)

Classify the final arterial system
The result of the preceding phase of the urban functional classification procedure should be a first approximation of an arterial system. At this point a reevaluation of the preliminary system is undertaken in order to define a final system of arterials.

The procedure used to determine the final arterial system will be highly dependent upon individual study circumstances. In cases where the preliminary arterial system is judged to be adequate, with relatively few facilities in question as to whether they logically function as arterials or collectors, this phase in the analysis may only involve a refinement of the application of the criteria described in step 'C' In cases where there are numerous questions regarding the proper functional classification of facilities (arterials versus collectors) , professional judgment and vision will be appropriate after considering all criteria and guidelines.

Classify the principal and minor arterial street systems
Step 'C' and 'D' were directed toward establishing the total system of arterials in the urban area. The next step is to identify an integrated system of principal arterials, with all remaining arterials designated as minor arterial streets. The principal arterial system, as defined earlier, comprises three categories of facilities: Interstate highways, other freeways and expressways, and other principal arterials. Since the first two of these categories consist of readily identifiable "facilities, the primary task described in this step entails the identification of the split between "other" principal arterials and minor arterial streets.

The criteria used in step 'C' for the designation of a total arterial system can be reapplied here to assist in this differentiation between "other" principal and minor arterial streets, as described below.

  1. Service to urban activity centers (traffic generators)
    In step C-1., all major generators which warrant arterial service were identified and mapped. A breakdown is now required to distinguish between those centers that should be served by the principal arterial system and those that require at least minor arterial street service. A principal arterial is considered to be offering service to a center when direct access is not further than about one-half to one mile from the facility, while for a minor arterial street, the suggested maximum range is from one-quarter to one-half mile.

    As mentioned previously, the rank ordering of traffic generators by quantitative and/or subjective criteria can assist in the allocation of functional responsibility. Generally, centers of regional significance should have principal arterial service, and community oriented centers usually should have at least minor arterial street service. The following list can serve as a guide in determining the generators to be served by the principal arterial system:

    1. Business districts of the central city(s) as well as those of larger satellite cities located within the urban area.

    2. Important air, rail, bus, and truck freight terminals.

    3. Regional retail shopping centers (those usually containing at least one major department store and generally selling goods, apparel and furniture, as opposed to convenience type of shopping goods).

    4. Large colleges, hospital complexes, military bases, and other institutional facilities.

    5. Major industrial and commercial centers.

    6. Important recreation areas such as regional parks, beaches, stadiums, and fairgrounds.

  2. System continuity
    The "building" of functional systems beginning with the principal arterial system should form, at the conclusion of each functional system addition, an integrated, continuous network throughout the area. Thus, the principal arterial system will be an integrated system which is continuous throughout the urbanized area (except as noted on page 111-15) and which also provides for statewide continuity of the rural arterial systems. The combined principal and minor arterial street systems will also form an integrated system. Likewise, when collectors, and finally locals, are added to the higher order systems the combinations at each stage are to be integrated systems. It should be understood that the minor arterials, collectors, and locals need not be integrated systems by themselves, but only in combination with the previously designated higher order system.

  3. Land use considerations
    Arterials can serve as buffers between incompatible land uses, and conversely, should avoid penetration of residential neighborhoods. Similarly, the configuration of the arterial system as a whole has a significant impact on land development policies and practices, although the magnitude of such impact is probably correlated with the relative significance of the arterial. In the extreme, controlled-access facilities serve best in separating land uses and generally have the most noticeable impact on land use.

    A pertinent land use consideration in the classification process is that of the degree of access to abutting land. The land access function of principal arterials is entirely subordinate to their primary function of carrying traffic not destined to land adjacent to the facility. Minor arterial streets, on the other hand, have a slightly more important land access function, though even for this class of facilities this is a secondary consideration.

  4. Spacing between routes
    It is difficult to define spacing criteria to assist in separating principal from minor arterials, since this factor has less bearing upon the differences that mark these two classes of roads than some of the other measures described in this section. In an ideal sense, spacing between principal arterials should be greater than spacing between minor arterial streets. Normally, minor arterial streets will be located between principal arterials.

    In the larger urbanized areas, the spacing of principal arterials may vary from less than one mile in the highly developed central business area to five miles or more in the sparsely developed suburban fringes. However, the nature of the land development pattern, and the associated travel patterns, in most urban areas will preclude the unqualified application of such an idealized rule.

  5. Average trip length
    Principal arterials should, as a general rule, serve trips which are significantly longer than those that are carried on the minor arterial street system. A qualitative (subjective) measure of trip lengths served by facilities is possible from a knowledge of the existing street and highway system and the routes generally used for long trips.

  6. Traffic volume
    The traffic volume criterion can be used here in a fashion similar to the procedure described in step C-6. However, a note of caution is warranted since the division between principal and minor arterials will be less subject to decision according to the amount of traffic carried on a facility than the split between all arterials and collectors. Because traffic volumes in the outlying portions of an urbanized area are generally lower than in the more densely populated central areas, the volume on a minor arterial street in the central city may be greater than the volume on a principal arterial in a suburban area. Thus, the volume of traffic carried by a facility should not be the controlling criterion in determining the proper system classification for a street, although it may be an important consideration.

  7. Control of access
    The access-control criterion is perhaps the most straight- forward to apply. Almost all facilities with full or partial control of access will fall within the principal arterial category. Partial access control is defined, for the purposes of this study, as the exercise of police power to limit access to a highway from abutting land to specified and controlled points. In a few instances such facilities may be determined to be functioning as minor arterial streets.

  8. Vehicle-miles of travel and mileage
    Upon completing the functional classification of arterials into the two basic categories, principals and minors, the cumulative vehicle-miles of travel carried by each class of facility in terms of cumulative mileage should be determined. These values should be compared with the general guidelines presented in Table 11-3. While exceptions are to be expected in a number of urban areas, an attempt should be made to describe the reasons for them where they do occur. If no substantive causes can be identified, consideration ought to be given to a re-examination of the functional classification as performed to this point.

    A typical plot for an urbanized area of cumulative urban street mileage versus cumulative vehicle miles served is shown in Figure 111-8.

  9. Substratify the principal arterial system
    Completion of step 'E' should produce a finalized breakdown between arterials and other facilities, as well as a stratification of arterials into principals and minors. The principal arterial system should be further divided into the three subcategories of Interstate highways, other freeways and expressways1, and other principal arterials. (Those facilities which are currently providing continuity between completed portions of the Interstate System should be designated as either other freeways and expressways or other principal arterials, as the case may warrant.)

    At this point in the development of a functionally classified system connecting links should be identified to provide continuity for rural arterials which intercept the urban area boundary.

  10. Classify collector and local streets
    With the designation of the arterial system, the remaining streets in the urban area will comprise those facilities which function as collectors and locals. It will be necessary to shift the scale of the analysis at this point in order to identify these classes of roads in terms of the individual streets which are in each functional category, the total amount of travel occurring on these classes of streets, and the total mileage they represent. Pertinent steps in the procedures described above, and the definitions and criteria presented earlier, should be applied to the fullest extent possible.

    The basic consideration here is that collector streets, which may have a relatively important land access function, serve primarily to funnel traffic between local streets, where the land access function is dominant, and the arterial system, where service to through traffic is of primary importance. In order to bridge this gap between locals and arterials, collectors must, and do, penetrate identifiable neighborhoods.

    With the identification of collector streets, all remaining facilities which have not been designated as arterials or collectors will necessarily fall within the local category. The extent of the collectors and locals, as measured by cumulative vehicle-miles of travel and mileage, should be computed with the generalized values presented in Table 111-3. Where significant differences exist, they should be noted and discussed.

"Future Year" Classifications
A functional classification for "future Year" system plans in urbanized areas can be developed as follows:

  1. Develop, in general concept, the pattern of future land uses in presently undeveloped areas within and around the city. Assumptions must be made (realistically) regarding major new commercial, industrial, institutional, and recreational developments as well as residential development. In the absence of a "future year" land use plan, guidance must come from the pattern of land use in the present urban area (particularly from recent growth, if any), from local knowledge of and development proposals, from the pattern of existing road network, from the effect of other transportation facilities, and from an examination of the terrain conditions in the area.

  2. Considering the above and the urban boundary criteria discussed on page 11-7, delimit the "future year" urban area boundary.

  3. Using the latest available functional classification as a base, delineate the principal arterial and minor arterial street networks within the future year urban area boundary. Included in these networks will be projected new facilities based on the land use plan or the assumption developed in (1) above and future systems plans developed by the urban planning process.

  4. Evaluate (for reasonableness) the extent of the projected mileage of new facilities developed in (3). Miles of arterials per square mile of area should be comparable to the rate in areas presently developed to a similar land use intensity. This miles-per-square-mile rate for facilities in the area of future urbanization should logically not be higher than the corresponding rate for the present urban area, since the latter includes the densely developed areas of the city. Attention should be given to providing an adequate limited access system for area mobility. In addition, consideration should be given to providing good intermodal connectivity.

  5. Projecting proposed locations for future collector and local streets in presently undeveloped areas may, in many cases, be impracticable. However, statistical estimates of future collector and local street mileage may be desired, particularly as a basic for projecting maintenance requirements. Statistical indices, such as a street-miles-per-square-mile rate, may be developed, based on existing developments at dwelling unit or population densities similar to that projected for the new area.

  6. Evaluate the adequacy of the overall classification plan to serve anticipated future year travel. The following questions, among others, should be considered: Does the pattern of principal arterials plus minor arterial streets provide adequate continuity for areawide movement? Are there sufficient limited access facilities to provide the proper channelization of trips? Does the proposed functional classification adequately support the intermodal transportation plan? Can anticipated future year capacity requirements be met within developable rights-of-way of the designated network or should additional arterials (one-way couplets, for example) be designated? Would such added arterials, in regard to their impact on the immediate environment, be representative of realistic proposals that might be implemented to satisfy local demand? Has the distinction between arterial and collector streets been properly and consistently defined?

  7. Develop the further subclassifications within the principal arterial street classes required to provide connecting links for the rural principal arterial and minor arterial systems as described on page 11-15.

Footnotes

  1. Note: Two special cases should be treated in the following manner: One-way streets should be classified individually, and their mileage and travel accumulated on an individual basis, not in pairs. Frontage roads should be classified independently of the controlled-access facility on which they abut. The classification of frontage roads, based upon the criteria presented in this manual should normally be in the collector or local category. Original is footnote 1 on page III-1.

  2. The designation of expressways should be in accordance with the American Association of State Highway and Transportation Officials (AASHTO) definition. Original is footnote 1 on page III-32.
Updated: 10/07/2013
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