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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation
FHWA Highway Safety Programs

3.0 Geography Definition

A clear and consistent definition of the terms "urbanized," "nonurbanized," "urban" and "rural" is needed to conduct target setting for urbanized and nonurbanized areas. A glossary of terms is provided in Appendix A.

3.1 Census Urban Areas

The U.S. Census Bureau defines and delineates the geographic boundaries for urban areas based on residential population after each decennial Census. Census-defined urban areas are used to summarize and report data collected by most Federal agencies. The Census definition of urban area includes urbanized areas of 50,000 or more population and urban clusters of at least 2,500 and less than 50,000 population. The Census Bureau uses the term "urban area" to refer to both urbanized areas and urban clusters collectively. This represents the most commonly used definition for urban areas and is based on a consistent methodology across all States.9

Beginning with the 2000 decennial Census, the Census Bureau has used a geographic information system (GIS) methodology to identify and construct the boundaries for urban areas based on aggregations of Census Blocks. Each urban area is built outward from a core of Census Blocks that meet an initial population density threshold; new blocks are added until the population density falls below a specified threshold, or until the urban area bumps against an adjacent urban area.

Figure 3.1 shows the 2010 urban area boundary for Columbus, Ohio. As shown, urban area boundaries defined by this process tend to be highly irregular in shape, often containing elongated “fingers” that follow major highways, as well as indentations and “holes” that represent areas with little or no residential population (e.g., urban parkland or industrial areas).

Figure 3.1 2010 Census Urban Area Boundaries in Columbus, Ohio

Figure 3.1  - 2010 Census Urban Area Boundaries in Columbus, Ohio

Source: Census

Census defined urban area boundaries may not coincide with the jurisdictional boundaries of incorporated cities or towns, counties, or even States. Parts of a particular urban area (e.g., Washington, D.C. or Philadelphia, Pennsylvania) can exist in two or more States.

3.2 FHWA Adjusted Urbanized Area Boundaries

FHWA and the Census Bureau differ in defining and describing 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 funding programs.

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. 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)).10

ccording 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, while the Census Bureau refers to “urban clusters”. These differences are summarized in Tables 3.1 and 3.2

The Federal government does not require Census urban boundary adjustments. States may adopt the Census boundaries as is, or adjust them for transportation planning purposes. The only official requirement is adjusted boundaries must include 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. The adjusted urbanized area boundaries can also include other areas that are “urban” in character but do not meet the Census Bureau’s population threshold (e.g., high density industrial or commercial areas, urban parks, etc.). The adjusted boundaries can also be expanded to ensure major roads do not alternate between urban and rural designations. This geography, called the “adjusted urbanized area” boundary, includes locations with a population of 50,000 or greater.

Figure 3.2 shows the 2010 Census defined urbanized area boundary (brighter green) overlaid on the adjusted urbanized area boundary (light green) for Columbus, Ohio. The adjusted urbanized boundary fills in several areas throughout Ohio that are more industrial than residential, and aligns the adjusted boundaries with major roads to minimize situations where a road alternates between an urban and rural designation.

Figure 3.2 Adjusted Urbanized Area Boundaries in Columbus, Ohio

The key geography used in this research is the FHWA adjusted urbanized area involving urbanized areas with a population of 50,000 or more. This is the geography proposed in the Safety PM NPRM with respect to urbanized and nonurbanized target setting. This definition is used by FHWA for any Federal reporting of urbanized areas in regulations or statistics, such as for the Urbanized Area Summaries on length and daily vehicle miles of travel (Table HM-71) and selected characteristics (Table HM-72) documented in the FHWA Highway Statistics Series.11

Tables 3.1 and 3.2 Summarize how Census and FHWA urban areas are defined by population range and note FHWA urban area boundaries can be adjusted.

Table 3.1 U.S. Census Bureau Urban Area Types Defined by Population Range

Census Bureau Area Definition Population Range
Urban Area 2,500+
Urban Clusters 2,500-49,999
Urbanized Area 50,000+

Source: FHWAHighway Functional Classification Concepts, Criteria and Procedures, 2013

Table 3.2 FHWA Urban Area Types Defined by Population Range

FHWA Area Definition Population Range Allowed Urban Area Boundary Adjustments
Urban Area 5,000+ Yes
Small Urban Area (From Clusters) 5,000-49,999 Yes
Urbanized Area 50,000+ Yes

Source: FHWAHighway Functional Classification Concepts, Criteria and Procedures, 2013

3.3 Other Urban and Rural Definitions

FARS definitions of urban and rural can vary from the urbanized area boundaries used by States. FARS uses the following definition for urban areas:12

An urban area is an area whose boundaries shall be those fixed by responsible state and local officials in cooperation with each other and approved by the FHWA, U.S. DOT. Such boundaries are established in accordance with the provisions of Title 23 of the USC. Urban area boundary information is available from state highway or transportation departments. In the event that boundaries have not been fixed as above for any urban place designated by the Bureau of the Census having a population of 5,000 or more, the area within boundaries fixed by the Bureau of the Census shall be an urban area.

NHTSA produces annual fact sheets on urban and rural crashes using the urban and rural definitions that FARS analysts report from each State. However, as FARS data are geocoded, the number of fatalities falling within the urbanized area boundary can be calculated using GIS. For consistency, beginning in 2016, NHTSA will begin reporting data using the FHWA adjusted urbanized areas geography.

Most States currently use other definitions for urban and rural. Common definitions include considering crashes occurring inside the boundaries of a municipality “urban”, or defining urban areas as those over a certain population threshold. The urban/rural definitions used for crash reporting forms are likely different from Federal definitions, so that is not be a reliable source for these computations.


See Section 6.


12FARS Users Manual 1975-2012).