|Functional Classification Home
|Please Note: The following illustrations are not yet available for publication.
- Figure III-1. Visitation vs. Equivalent Population for Ranking Recreation Generators
- Figure III-2. Graphic Ranking and Grouping of Travel Generators (for a typical site)
- Figure III-3. Plot of Cumulative Road Mileage Versus Cumulative Vehicle Miles Served
FHWA Functional Classification Guidelines
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."
SECTION III - SUGGESTED PROCEDURES FOR
RURAL, SMALL URBAN AREA AND
URBANIZED AREA CLASSIFICATION
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
(Illustration scanned, but not yet prepared for publishing)
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:
- 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.
- 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.
- 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-3. Visitation VS. Equivalent Population For Ranking Recreation Generators
(Illustration scanned, but not yet prepared for publishing)
- 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.
- 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.)
- Delineate all presently designated routes of the Interstate highway system.
- 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.
The size of the travel generators being connected has been visually symbolized on the map. The predominant travel distance and size of the tributary area or "travel shed" can be inferred visually from the size of centers served, their spacing and orientation, and the size and shape of traffic flow bands of traffic maps.
Judgment must be exercised in determining which, among all possible connections, should be made, especially when dealing with medium-sized and smaller centers. It will be helpful to keep in mind that this procedure is based on an indirect and inferential approach to the traffic attraction between centers. Therefore, the traffic flow map will help to indicate which, of all possible connections, is the most significant for the level or size of center being considered. When medium and small-sized centers are under consideration, a connection with the nearest larger center is usually more significant than a connection with a center of equal size. Where alternatives are equal in terms of mileage, the most heavily traveled and the better improved route should normally be selected.
The termini for the routes being added to each system should be selected so that a continuous system is always maintained (i.e., each route is connected to routes of the same or higher level system).
- Determine the total length of the rural principal arterial system in accordance with the system characteristics and the guide on system mileage extent in Section II, page II-11.
- Determine the lower size limit of population centers to be served as a group by the minor arterial system. In the criteria for this system, the diminishing returns concept is mentioned. This means that in adding routes to a system, a point is reached at which the rate of increase in mileage begins to exceed markedly the rate of increase of highway service, indicating that the lower limit of the system under consideration has been determined. Figure 111-3, on which cumulative system mileage has been plotted against cumulative service as measured by vehiclemiles of travel, is an illustration of this concept.
The concept illustrated in Figure 111-3 can be applied during the classification process by visual evaluation of the system map in conjunction with basic data on traffic flow and population. The following considerations, used in conjunction, should apply. First, as indicated in Section II, page 11-11, a combined mileage of rural principal arterial and minor arterial classes of between 7 and 10 percent of total rural mileage is the normal maximum extent. considering this as the limit being approached, then: (1) Would adding routes to serve a next group of smaller generators result in adding a considerable mileage of routes carrying, as a group, substantially less traffic than routes already added? (2) Is the radius of traffic attraction of this next group of smaller generators, as implied by their size, their distance from larger generators, or by traffic flow data, substantially less than that of places already served? If the answer to either question is yes, then a logical lower limit of the minor arterial system has been reached, as far as service travel generators is concerned.
|Figure III-3. Plot of Cumulative Road Mileage Versus Cumulative Vehicle Miles Served
(Illustration scanned, but not yet prepared for publishing)
"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.
- Add such other routes to the minor arterial system as are required by the defined system characteristics. Such routes will include:
- (Service to corridor movements with trip lengths and volumes equivalent to those of routes already added, as determined from traffic flow maps.
- Service to all areas of the State, with spacing of routes at reasonably consistent intervals, as tempered by consideration of population density.
- Such additions as are clearly needed for adequate statewide continuity (but only where significant travel patterns serve to justify them).
- 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:
- 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.
- Where one major facility is a parkway from which commercial vehicles are excluded.
- Where the total traffic volume cannot practicably be handled by one facility.
- Where one facility is a toll road.
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.
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:
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:
- Location of population centers (including county seats) not already served by the higher systems.
- 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.
- Location of any heavier-than-average corridor movements within the county, from traffic flow data.
- Location of existing freeway interchanges or important river crossings that may be key location controls with regard to the collector system.
- Rural population and land-use distribution within the county as regards uniform or nonuniform density of development.
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.
- 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.
- Link the more important local traffic generators with nearby population centers or with this or a higher system.
- Serve corridor movements with traffic volumes and trip lengths comparable to those of major collector routes already selected.
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
|Central business district
|Urban (central city except CBD)
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:
- 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.
- considering the above and the urban boundary criteria discussed on page 11-7, delimit the "future year" urban area boundary.
- 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.
- 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.
- 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.
- 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?
- 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.
For the next part of Section III, Suggested Procedures for Rural, Small Urban Area and Urbanized Area Classification, please go to the next page.
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- 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.