|Functional Classification Home
|The following illustrations are not yet available for publication.
- Figure III-4. Distribution of Development Intensities for a Typical Urbanized Area
- Figure III-5. Minimum Theoretical Arterial Spacing Required to Accommodate Arterial Travel Demand at Route Capacity
- Figure III-6. Average Trip Lengths on Segments of an Urban Stret System (for a typical area about 1,000,000 population)
- Figure III-7. The Relationship Between Travel Density and Cumulative Mileage on the Arterial System (in a typical large urbanized area)
- Figure III-8. Plot of Cumulative Urban Street Mileage Versus Cumulative Vehicle Miles Served (for a typical urbanized area)
FHWA Functional Classification Guidelines
Part III Continued
FOR URBANIZED AREAS
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):
- Determine and map the urbanized area boundary.
- Map the road network.
- Perform a preliminary classification of the total arterial system.
- Classify the final arterial system.
- Classify the principal and minor arterial street systems.
- Substratify the principal arterial system.
- Classify collector and local streets.
A. 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.
B. 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.
C. 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.
- 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
(Illustration scanned, but not yet prepared for publishing)
- 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.
- 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.
- 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
|Central business district
|Urban (central city except CBD)
|Lowest density development
|Figure III-5. Visitation VS. Minimum Theoretical Arterial Spacing Required to Accommodate Arterial Travel Demand at Route Capacity
(Illustration scanned, but not yet prepared for publishing)
- 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
(Illustration scanned, but not yet prepared for publishing)
- 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
(Illustration scanned, but not yet prepared for publishing)
- 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.
- 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 Vehicle Miles Served
(Illustration scanned, but not yet prepared for publishing)
D. 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.
E. 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:
- Business districts of the central city(s) as well as those of larger satellite cities located within the urban area.
- Important air, rail, bus, and truck freight terminals.
- 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).
- Large colleges, hospital complexes, military bases, and other institutional facilities.
- Major industrial and commercial centers.
- 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.
F. 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.
G. 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:
- 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.
- 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 (1) above and future systems plans developed by the urban planning process.
- 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.
- 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 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?
- 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 previous part of Section III, Suggested Procedures for Rural, Small Urban Area and Urbanized Area Classification, please go to the previous page.
- 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.