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Highway Functional Classification Concepts, Criteria and Procedures

Section 6. Urban Boundaries

6.1 Introduction

Many Federal transportation programs and policies rely upon a clear and well-documented distinction between urban and rural areas. Urban and rural areas are explicitly defined by the Census Bureau according to specific population, density and related criteria. From these technical definitions, irregularities and boundaries that are separated from or inconsistent with transportation features may result. For transportation purposes, States have the option of using census-defined urban boundaries exclusively, or they may adjust the census-defined boundaries to be more consistent with transportation needs. States, in coordination with local planning partners, may adjust the urban area boundaries so fringe areas having "…residential, commercial, industrial, and/or national defense significance" (as noted in the December 9, 1991 Federal-Aid Policy Guide), are included.

The authority to establish the geographic definitions is set forth in Section 101(a) of Title 23 U.S.C. and subsequent guidance has been provided in 23 CFR 470 and in FHWA policy documents.

Reasons for adjusting urban area boundaries for transportation planning purposes often relate to a need for consistency or geographic continuity. For example, it may be logical to include, as part of an urban area, a roadway that is used by urban residents but is located just outside the official Census Bureau urban area boundary. Or, it may make sense to designate as urban a rural pocket in the middle of an urban area (or to address alternating patterns of rural and urban-designated areas). Additionally, large, low density land uses on the urban fringe that serve the urban population such as airports, industrial parks, regional shopping centers and other urban attractions may also be included in an urban area.

On October 14, 2008, FHWA issued the memorandum "Updated Guidance for the Functional Classification of Highways" which stated, "Functional classification should not automatically change at the rural/urban boundary." This extended the 1991 Addendum to the 1989 guidance Highway Functional Classification: Concepts, Criteria and Procedures, which provided "greater flexibility for deciding on an appropriate place for changing the functional classification when rural routes cross an urban boundary." The 2008 memorandum proposed further study of functional classification and urban area boundary adjustment which led to this document.

This section is intended to assemble and complete all previous policy given by FHWA for establishing urban area boundaries. It has three main objectives:

  1. To provide a clear definition of adjusted urban area boundaries and other related boundaries
  2. To define a set of technical and administrative processes by which States, working in conjunction with local planning partners, could develop adjusted urban areas based upon urban areas as defined by the US decennial census
  3. To establish data delivery protocols from the States to FHWA

6.2 Defining Urban and Rural

The concept of adjusted urban areas has evolved since the issuance of the Federal guidance on the topic in Chapter 4 of FHWA's Federal-Aid Policy issued in December 1991.

The terms "urban" and "rural" mean different things to different people, and in many cases, their definitions differ depending upon the context in which they are used. At their core, the concepts of urban and rural are clear; urban areas are considered to have dense development patterns, while rural areas are considered to have sparse development patterns (see Figure 6-1). What has changed over the years, however, is the terminology used and the technical definitions of "dense" and "sparse".

Figure 6-1: Prototypical Urban and Rural Areas



This figure shows side-by-side photographs of a busy Urban Interstate, on the left side and an empty rural Other Principal Arterial, on the right side.

Source: CDM Smith

6.2.1 Census Definitions

For the 2010 Census, the Census Bureau classified as urban, all territory, population, and housing units located within urbanized areas (UAs) and urban clusters (UCs), both defined using the same criteria. The Census Bureau delineates UA and UC boundaries that represent densely developed territory, encompassing residential, commercial, and other non-residential urban land uses. An urban area comprises a densely settled core of census tracts and/or census blocks that meet minimum population density requirements, along with adjacent territory containing non-residential urban land uses as well as territory with low population density included to link outlying densely settled territory with the densely settled core. To qualify as an urban area, the territory identified according to criteria must encompass at least 2,500 people, at least 1,500 of which reside outside institutional group quarters.

For the 2010 Census the urban and rural classification was applied to the 50 states, the District of Columbia, Puerto Rico, American Samoa, Guam, the Commonwealth of the Northern Mariana Islands, and the U.S. Virgin Islands.

According to definitions in 23 U.S.C. 101(a)(33), areas of population greater than 5,000 qualify as urban for transportation purposes in contrast to the Census Bureau's threshold of 2,500.

For classification purposes, the Census Bureau identified two types of urban areas for the 2010 Census:

Urbanized Areas (UAs)-An urbanized area consists of densely developed territory that contains 50,000 or more people. The Census Bureau delineates UAs to provide a better separation of urban and rural territory, population, and housing in the vicinity of large places.

Urban Clusters (UCs)-An urban cluster consists of densely developed territory that has at least 2,500 people but fewer than 50,000 people. The Census Bureau first introduced the UC concept for Census 2000 to provide a more consistent and accurate measure of urban population, housing, and territory throughout the United States, Puerto Rico, and the Island Areas.

A full description of the final 2010 Census urban area delineation criteria can be found in the August 24, 2011, Federal Register (76 FR 53030): Additional information regarding the 2010 Census urban area program can be found:

In general, this territory consists of areas of high population density and urban land use resulting in a representation of the "urban footprint." Rural consists of all territory, population, and housing units located outside of UAs and UCs.

Geographic entities, such as metropolitan areas, counties, minor civil divisions (MCDs), places, and census tracts often contain both urban and rural territory, population, and housing units.

6.2.2 FHWA Definitions

There are differences in the way FHWA and the Census Bureau define and describe urban and rural areas. The Census Bureau defines urban areas solely for the purpose of tabulating and presenting Census Bureau statistical data. A number of Federal agency programs use the census definitions as the starting point (if not the basis) for implementing and determining eligibility for a variety of their funding programs.

According to 23 U.S.C. 101(a)(33), areas of population greater than 5,000 can qualify as urban, in contrast to the Census Bureau's threshold of 2,500. There are also differences in the terminology used to describe sub-categories of urban areas. FHWA refers to the smallest urban area as a Small Urban Area[13], while the Census Bureau refers to Urban Clusters. This and other differences are presented in Table 6-1 and Table 6- 2.

Table 6-1: US Census Bureau Urban Area Types Defined by Population range

Census Bureau Area Definition

Population Range

Urban Area


Urban Clusters


Urbanized Area


Table 6-2: FHWA Urban Area Types Defined by Population Range

FHWA Area Definition

Population Range

Allowed Urban Area Boundary Adjustments

Urban Area



Small Urban Area (From Clusters)



Urbanized Area



Federal transportation legislation allows for the outward adjustment of Census Bureau defined urban boundaries (of population 5,000 and above) as the basis for development of adjusted urban area boundaries for transportation planning purposes, through the cooperative efforts of State and local officials. By Federal rule, these adjusted urban area boundaries must encompass the entire census-designated urban area (of population 5,000 and above) and are subject to approval by the Secretary of Transportation (23 USC 101(a) (36) - (37) and 49 USC 5302(a) (16) - (17)).

For the purposes of the boundary adjustment process, the term "adjusted urban area boundaries" refers to the FHWA boundary adjustment process in all areas of 5,000 population and above.

During the time between the release of the Census Bureau boundaries and the formal approval of the new adjusted boundaries, the previously-developed and approved adjusted urban area boundaries remain in effect. For FHWA and State DOT planning purposes, if a State DOT chooses not or is unable to adjust the urban area boundaries, the most recent unadjusted census boundaries will take effect. This could cause a roadway previously considered to be urban to now be considered rural, which may affect Federal aid funding eligibility.

To avoid this situation, States are encouraged to work with their FHWA Division Office and their local planning partners to go through the process of developing the adjusted urban area boundaries within the recommended timeframe.

6.3 Relationship to Functional Classification

While the urban/rural designation is independent of the functional classification, it is important to recognize that the adjusted urban area boundary is a significant factor in developing the functional classification of a road in an urban/rural context.

Recent changes to FHWA policy have normalized[14] the concepts of urban boundaries and functional classification to improve consistency. The seven functional classifications each for urban and rural areas create 14 possible combinations of functional class and area type. As an example, a roadway classified as a Minor Arterial that happens to be in an urban area has a combined classification of Urban Minor Arterial. There is no change in the definitions of the functionally classified roads; nor does this in any way change the eligibility of rural and urban-classified roads for Federal programs and policies, or how highway statistics are reported.

This change in policy provides an opportunity to clarify how functional classifications at the boundaries of urban/rural areas should be treated. The previous practice in some States of automatically changing the functional classification of a route that crosses into or out of an adjusted urban area boundary can be phased out and eliminated. Upgrading due to an actual change in function should be the operative criterion.

Special attention should be paid to locations at which roadways and boundaries are in close proximity. The adjusted urban area boundary should be designed to eliminate or minimize a roadway's snaking in and out of the boundary. In these cases, as the boundary is adjusted, it needs to be clearly defined that the road is either in or out. This adjustment serves to maintain consistent designation of these peripheral routes and avoids the situation of a roadway alternating between urban and rural designations. Special care should be taken when developing the boundary so that spatial consistency is maintained with the roadways and associated attributes.

Roads that define a boundary should be considered consistently urban or rural, and it is strongly recommended that these roadways be carefully evaluated before they are included in or out of the adjusted urban area boundary. For example, in Figure 6-2, Plympton Street (a Major Collector) defines the adjusted urban area boundary and is considered to be an Urban Major Collector, while Plymouth Street (a Local Road) is considered to be an Urban Local Road.

Figure 6-2: Example of Roadway Coinciding with Adjusted Urban Area

Map that shows the boundary of an adjusted urban area. The boundary exactly follows a street called Plympton Street, and it is an Major Collector in an urban area.

Source: CDM Smith 2012; Data provided by Massachusetts DOT

6.4 Developing Adjusted Urban Area Boundaries

This section outlines a series of recommended technical and procedural steps to develop adjusted urban area boundaries. These tasks are typically conducted through a collaborative effort between State DOTs and local planning partners. The process begins with the release of the urban area boundaries by the Census Bureau and concludes with the approval of the appropriate FHWA Division Office. Overall, the process typically takes between six months and a year to complete from the time that the census boundaries are released.

As described previously, there is no requirement to adjust the census urban boundaries. States may adopt the census boundaries as is, or they may adjust them for transportation planning purposes. The only official requirement is that an adjusted boundary includes the original urban area boundary defined by the Census Bureau in its entirety. In other words, any adjustment must expand, not contract, the Census Bureau urban area boundary.

6.4.1 Adjusted Urban Area Boundaries - Technical Tasks

The first step in defining adjusted urban area boundaries is to obtain the census urban area geospatial boundary files from the Census Bureau. These files are available from FHWA's HEPGIS website or from the Census Bureau in a variety of GIS-compatible formats, including Arc/Info export, Arc View shape file and Arc/Info format. Historical cartographic boundary files from previous censuses are available for download at:

These urban area boundary files should be edited in GIS. Additional GIS layers should also be gathered from the same year as the decennial census (e.g., 2010) or of similar vintage (see Figure 6-3). Potentially useful GIS layers include:

Figure 6-3: 2000 Census Urban Cluster and Urbanized Areas (Ohio and Vicinity)

Map that shows the location of Urban Clusters and Urbanized areas in the state of Ohio and parts of Indiana and West Virginia.

Source: 2000 US Census

6.4.2 Consideration Factors for Adjusting Urban Areas

When adjusting the urban areas, a variety of factors should be considered. The list below describes these factors and includes an example for each. All examples are courtesy of the Arizona or Massachusetts departments of transportation.

Figure 6-4: Example Original Urban Area

Map of Pinetop-Lakeside, Arizona. The area's boundary is clearly shown around the area. The boundary has not been expanded beyond the Census boundary.

Source: Arizona DOT;

Figure 6-5: Example Single Contiguous Area

Map of Page, Arizona and it shows a single, contiguous area boundary, without any holes or discontinuities.

Source: Arizona DOT;

Figure 6-6: Example Area Expanded to Cover Air Force Base

Map that shows an example area whose boundary has been expanded to encompass an entire municipality.

Source: Map created by CDM Smith, using data provided by Massachusetts DOT and US 2000 Census.

Figure 6-7: Example Area Expanded to Include Industrial Area

Map that shows an example area whose boundary has been expanded to encompass a large industrial development.

Source: Arizona DOT; with overlay graphic by CDM Smith to identify industrial plant.

Figure 6-8: Example Boundary Adjusted to Align with Major Roadway

Map that shows an example area whose boundary has been shifted to follow a physical boundary, a road, rather than the boundary of the jurisdiction.

Source: Source: Arizona DOT;

Figure 6-9: Example Boundary Adjusted for Simplicity

Map that shows an example area whose boundary has been re-aligned to be straighter than the previous boundary.

Source: Arizona DOT;

Additional recommendations regarding the adjustment of the urban area boundaries include:

6.5 Adjusted Urban Area Boundaries - Procedural Tasks

If States and their local partners choose to adjust the urban area boundaries, then they must be reviewed, at a minimum, in conjunction with the census urban area boundary release.[15] FHWA recommends that this process be completed within 1 year of the release of the census urban area GIS datasets. FHWA considers a State's DOT, working with the appropriate local government entities, to be the authority during this process and relies upon State DOTs to take an active leadership role.

6.5.1 Risk Factors to Urban Area Adjustment Schedule

There are several risk factors that could potentially arise and impact the amount of time it takes to complete the adjustment process. Therefore each State should develop a carefully planned approach for addressing these potential risk factors, which include:

6.5.2 Urban Area Adjustment Schedule

FHWA Division Offices will correspond with State DOTs to launch the effort of developing the adjusted urban area boundaries. This transmittal is expected to be delivered soon after the Census Bureau releases its urban area boundaries, which typically occurs about 12 to 18 months following the decennial census. FHWA's transmittal will remind the State DOTs of their responsibilities; include notification of the availability of the Census Bureau's urban area boundary files; and provide information regarding how and when the updated boundary data should be submitted.

Figure 6-10 and the list that follows present a good practice level of procedural steps that should be completed within 12 months of the release of the Census Bureau's urban area boundary files.

Example Area Expanded to Include Developing Urbanization. This figure is a map that shows an example area whose boundary has been expanded to encompass some recent urbanization. The boundary adjustment follows property lines rather than a road.

Figure 6-10: Good Practice Level of Procedural Steps for an Urban Boundary Update Process

  1. Mobilize the Urban Area Boundary Adjustment Process
    1. Acquire newly developed urban area boundaries from US Census. Obtain the latest decennial census urban area boundaries from the Census Bureau.
    2. Form a team to guide the urban area boundary update process. Staff the team with FHWA Division personnel, along with State and regional transportation planners who have a vested interest in the final delineation of the boundaries. Individuals with experience in functional classification, Federal transportation funding, highway design, traffic operations and the metropolitan transportation planning process should have a role in this process. This review team should be responsible for reviewing draft adjusted urban area boundary submittals from local planning partners.
    3. Generate data, maps, etc. for use by local planning partners. Incorporate urban area boundaries from the census into data and maps that that are relevant to local planning partners. These may include statewide, district, county and municipal scales.
    4. Contact local planning partners. Contact the impacted local planning partners to explain the task at hand and request their participation. For Urbanized Areas contained and/or very proximate to metropolitan planning areas, the MPO should be a key partner. For Urban Clusters, regional planning agencies, counties and/or local municipalities should be consulted. However, for many of these urban areas, additional effort may be required to properly engage these partners. In these instances, it is appropriate for State DOTs to make urban area adjustments in these areas. Finally, in some instances, regional transit service providers should also be consulted to understand their short-term routing plans.
  2. Work with Local Planning Partners in the Adjusted Urban Area Boundary Update Process
    1. Deliver data and documents to local planning partners. Share the original decennial census-based urban boundary maps and/or GIS data (including both Urbanized Areas and Small Urban Areas) with the local planning partners. In addition, to inform the partners and the process more completely, it helps if maps and/or GIS data representing both the previous unadjusted and adjusted urban area boundary are shared in a timely manner. This transmittal should include specific instructions in terms of data formats, spatial accuracy, update processes and expected completion dates, as well as this guidance document. In-person or video conference meetings are encouraged to enhance communication and mutual understanding. Creation of adjusted urban area boundaries should follow each State's GIS data editing and quality control procedures (e.g., issues of scale) and performed by qualified GIS users.
    2. Work with local planning partners. As necessary, each State DOT will need to work with the local planning partners to ensure that the urban area adjustment process is meeting their expectations. Close collaboration with MPOs is extremely important, and regional workshops hosted by MPOs can be very valuable in ensuring there is a common understanding of the process and schedule. While the exact details surrounding information exchange may vary from state to state, the expectation is that local planning partners will review the US census urban area boundaries in the context of the existing adjusted urban area boundaries (based upon the previous census) and determine the extent to which the boundaries should be adjusted for transportation planning purposes. The local planning partners should submit a set of proposed adjustments to the current US Census urban area boundaries in their area to their State DOT.
  3. Make Adjusted Urban Area Boundary Changes
    1. Gather, review and incorporate proposed changes from local planning partners. As local planning partners submit their recommendations for adjusted urban area boundaries, the State DOT must review the proposed adjustments to ensure that they are reasonable. At the very least, the DOT must ensure that no territory considered urban by the Census Bureau be left out of the adjusted urban area boundary. In addition, the State DOT should review all proposed adjusted urban area boundaries paying particular attention to locations where the adjusted urban area boundaries are co-located with another feature such as a roadway, a municipal boundary or a hydrographic feature. Some follow-up meetings may be necessary to resolve issues discovered by the DOT. The updated GIS adjusted urban area boundaries need to be incorporated into the master urban boundary layer and subjected to the DOT's GIS quality control checks with the metadata for the layer updated.
    2. Submit draft adjusted urban area boundary information to FHWA Division Office. Once the State DOT has successfully reviewed and concurred with all recommend adjusted urban area boundaries, the State DOT should submit the draft final adjusted urban area boundaries to its FHWA Division Office for final approval. The specific format of data delivery should be worked out between the State DOT and their FHWA Division Office. Various geospatial formats will be acceptable, and as developed, FHWA systems such as HPMS or HEPGIS may be used. As a final resort, hard copy maps at a scale sufficient to identify the adjusted urban area boundaries can be submitted.
    3. Incorporate adjusted urban area boundary changes into Enterprise Systems. Once FHWA has approved the adjusted urban areas, the State DOT should incorporate the adjusted urban area boundary changes into the enterprise geospatial database systems that house the official record of the adjusted urban area boundaries. States are required to submit their adjusted urban area boundaries to FHWA when changes are made to the boundaries. In most cases, this submittal should only occur once after the State has completed its adjustment process.

Table 6-3 presents key milestones for the overall development and submittal process (for example, using submitted data based upon the 2010 US Census data.

Table 6-3: Key Milestones for Development and Submittal of Adjusted Urban Area Boundaries


Months Following Decennial Census Data Release (CDR)

Census releases urban area boundaries and FHWA issues transmittal letter

Month 24

Begin adjusted urban area boundary update process

Month 24

DOT works with planning partners to define adjusted urban area boundaries

Month 27-Month 33

Provide draft final data and/or maps to FHWA Division Office for review

Month 34

DOT incorporates updates

Month 35

DOT submits adjusted urban area boundaries via annual HPMS submittal

Month 36

Each State should submit only boundaries for the HPMS submittal that have been approved by their FHWA Division Office.

Table 6-4 lists the attributes that are required within the FHWA geospatial database.

Table 6-4: Geospatial Database Required Attributes

Field Name



Year for which the data apply


Census urban code


Urban name


Census population ("recalculated" based upon the adjusted urban area boundary)


Census land area (in square miles)


Polygon feature

6.6 Adjusted Urban Area Boundaries - Data Transmittal Process

Each State DOT should coordinate with its local FHWA Division Office to discuss the data transmittal process. To the extent possible, all draft final boundaries should be submitted electronically in the form of GIS data and/or PDF maps. If GIS data are provided, appropriate metadata delineating the spatial accuracy, projection and definition/domain of all attributes should also be provided, as well as supporting documentation that briefly describes the process by which the boundaries were adjusted. In addition, each adjusted urban area boundary should be a single (multi-part, if necessary) polygon GIS feature. Feature names and codes should follow Federal Information Processing Standards (FIPS) conventions as well as any applicable State naming and coding standards.

Updated: 10/07/2013
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