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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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Appendix C
Analysis of Density Assumptions and Feasibility of Committee Scenarios

In this appendix, the plausibility of the committee’s scenarios of increasing compact development by 2050 is analyzed through a comparison of the scenarios with trends in residential development. The purpose of this exercise is to understand how substantial the changes from the trends would have to be to achieve a doubling of density of either 25 percent (Scenario 1) or 75 percent (Scenario 2) of new residential development. Two different data sources are available with which to measure trends in development patterns, and there are different ways of measuring the density of residential development. The committee used both of the main data sources and two different ways of measuring density to test the assumptions behind its scenarios. The plausibility of the scenarios is tested by simply calculating how much change would be required from the trends to achieve the projected level of compact development by 2050 and then asking whether such change appears achievable. Conducting this analysis requires first a review of data sources and density measures. The second section uses the data sources and density measures to provide a summary of development trends. The third section presents the comparison between the trends and the scenarios, examines various ways in which the scenario target densities could be achieved, and provides a summary of the committee’s conclusions.

DATA SOURCES AND DENSITY MEASURES

As discussed in Chapters 1 and 2, residential density is typically calculated as the number of persons, households, or dwelling units (DUs) divided by some unit of land area [e.g., acre, square mile (mi2)].

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

Ideally, the denominator should be a measure of net residential acreage to closely match the land use, but such data are not readily available.1

Data Sources

The principal data source for the numerator of the density calculation is the decennial U.S. Census of Population and Housing. As discussed in Chapter 2, two national sources of data can be used to track changes in land use—the denominator—over time, each with its pros and cons. The first is the National Resources Inventory (NRI), collected by the Natural Resources Conservation Service of the U.S. Department of Agriculture. The NRI surveys hundreds and sometimes thousands of sample points in each U.S. county and reports, among other data, the acres of land in each of a series of land-cover categories. The NRI combines two of these categories, urban and built-up areas and rural transportation land, to derive an estimate of developed land (see Box C-1 for detailed definitions). For the purposes of this study, the NRI’s landcover data were aggregated to correspond to U.S. census designations for metropolitan areas as defined in 1999.2 For the density calculation, the NRI’s urban and built-up land category was used as the denominator— referred to in this appendix as urban acres—and intercensal estimates of housing units at the metropolitan level as the numerator.

The advantage of using the NRI data is that they provide true landcover-based estimates of aggregate density, excluding land—even that within city limits—upon which development has not occurred. The NRI data also include urban and built-up land beyond census-defined “urbanized areas” where a very large share of recent development has

1

Net residential acres include only residential land. Nonresidential land uses are excluded, as are local streets and parks (Downs 2004). Gross residential acres, by comparison, include all land within a geographic area, regardless of its uses. Unfortunately, there is no simple or well-established way to relate these two density measures because conditions vary widely from one location to another.

2

The most recent year for which detailed inventory data from the NRI are available is 1997.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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Box C-1

DEFINITION OF DEVELOPED LAND

Developed land: A combination of land cover/use categories, including urban and built-up areas and rural transportation land. Urban and built-up areas consist of residential, industrial, commercial, and institutional land; construction sites; public administrative sites; railroad yards; cemeteries; airports; golf courses; sanitary landfills; sewage treatment plants; water control structures and spillways; other land used for such purposes; small parks (less than 10 acres) within urban and built-up areas; and highways, railroads, and other transportation facilities if they are surrounded by urban areas. Included are tracts of less than 10 acres that do not meet the above definition but are completely surrounded by urban and built-up land. Two size categories are recognized in the NRI: (a) small built-up areas of 0.25 acre to 10 acres and (b) large urban and built-up areas of at least 10 acres. In 1997, both size categories accounted for 78 percent of total developed land.

Rural transportation land consists of all highways, roads, railroads, and associated rights-of-way outside urban and built-up areas, including private roads to farmsteads or ranch headquarters, logging roads, and other private roads, except field lanes. In 1997, this category accounted for the remaining 22 percent of total developed land.


Sources: NRCS 2002; NRCS 2003.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

occurred, but at densities below the census’s threshold for identifying urbanized blocks (Downs 2004). The main disadvantage of the NRI is that it does not provide accurate estimates of land cover below the county level. It therefore cannot be used either to describe the pattern of development within regions (e.g., the continuity of development) or to provide a fine-grained breakdown of development density within a metropolitan area (e.g., the share of housing built above 8 DUs per acre).

The alternative data source is the decennial U.S. Census of Population and Housing, which reports (among many other variables) the number of housing units and the amount of land area for the entire United States. The advantage of these data is that they have greater spatial accuracy than the NRI can provide for small areas, with reports available down to small blocks based on the 100 percent enumeration of housing units. They can therefore be used to classify small areas—including census tracts, as is done here—into a range of density categories for core-based statistical areas (CBSAs).3,4 By doing so, it is possible to estimate the amount of housing in each metropolitan and micropolitan area that is built at what are presumably the lowest density levels, as

3

Data on housing, measured at the census tract level, are drawn from the National Neighborhood Change Database, a special tabulation of data from the 1970 through 2000 censuses that allocates population and housing to the boundaries of the 2000 census. The 2000 census Geocorr application from the University of Missouri was used to collect data on the land area of all U.S. census tracts. Then, for each census tract, the number of housing units was divided by the total land area to yield an average (gross) density figure. Each tract in 1990 and 2000 was allocated to a density category (e.g., <1 acre, 1 to 1.9 acres), summing the number of housing units and land area in tracts in each density range.

4

A CBSA is defined as a geographic entity consisting of a county or counties associated with at least one core (urbanized area or urban cluster) of at least 10,000 in population, plus adjacent counties having a high degree of social and economic integration with the core as measured through commuting ties with the counties containing the core (OMB 2000). Metropolitan and micropolitan statistical areas are the two categories of CBSAs. A metropolitan statistical area is a CBSA associated with at least one urbanized area that has a population of at least 50,000. A micropolitan statistical area is a CBSA associated with at least one urban cluster that has a population of at least 10,000 but less than 50,000. Both metropolitan and micropolitan statistical areas make up the central county or counties containing the core, plus adjacent outlying counties having a high degree of social and economic integration with the central county as measured through commuting.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

well as at higher density levels.5 The disadvantage of census tract–level data, however, is that such data are a gross measure of available land within census tracts (i.e., do not net out land uses for purposes other than residential development), thus in all likelihood underestimating the density of development in the most extensive tracts. For this reason the census data are referred to as gross acres in this appendix. Because of these pros and cons, both data sources are used here to examine density levels in light of recent development trends and how much change from current trends the committee’s scenarios imply.

Density Measures

One other distinction should be made. Two methods of defining densities are possible. The conventional method (A) simply divides total DUs (or persons) by total land area to get the density of the average acre of land. The alternative method (B) is to weight developments of different densities by the number of DUs they contain to get the density of the average DU (see Box C-2 for an example). The differences and similarities between the two sources of land data and the two methods of calculating density are illustrated in Tables C-1 through C-3. Table C-1 provides a summary of changes in density patterns from 1987 to 1997 and 1990 to 2000 on the basis of density figures from the NRI and the census, respectively. Tables C-2 and C-3 provide more detail on each. Method A for calculating density can be used with both the NRI and the census measures of land area, but Method B is possible only with the more spatially detailed census data.

Densities are higher when calculated with land use data from the NRI than with those from the census, presumably because the urban acres reported by the NRI include less undeveloped land than do the gross acres reported by the census. For example, the average density

5

It should be noted, however, that census tracts vary in size, becoming larger at metropolitan area boundaries.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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Box C-2

TWO METHODS OF DEFINING DENSITIES: AN EXAMPLE

Two methods of defining densities are used in this study—one weights by population, the other by acres. A simple example illustrates the difference. Assume a hypothetical metropolitan area of 10 acres in which 5 people live. But 1 person lives on the periphery on a 9-acre lot, while the other 4 people live in the metropolitan center on ¼-acre lots.

The density to which the average acre is developed is 5 people/ 10 acres = 0.5 person per acre. However, the average person in the metropolitan area does not experience a density of 0.5 person per acre but something much higher. That average is ( person × 4 persons per acre) + ( person × 1 person/9 acres) = 3.20 + 0.02 = 3.22 persons per acre.

The two measures differ because the lot sizes vary within the metropolitan area and because the former measure weights the density by acres, while the latter weights the density by persons. If everyone in the hypothetical metropolitan area lived on a 2-acre lot, for example, the average density would be 0.5 person per acre under either approach.

Both approaches are legitimate. The population-weighted density measure provides a better sense of the density to which the average metropolitan resident is exposed rather than the density to which the average acre in the metropolitan area is developed. The former is more likely to reflect the local circumstances in which the typical person lives. In the hypothetical metropolitan area, for example, it would be misleading to say that the typical resident behaved as if he or she lived in a neighborhood of 0.5 person per acre when four-fifths of the population lives at 4 persons per acre.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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TABLE C-1 Summary of Recent Trends in the Density of Development Patterns Based on the NRI and the U.S. Census

Based on NRI data, 1987–1997 (urban acres)

 

Density of average acre (Method A)

 

Density of average acre: 1987

1.86 DUs/acre

Density of average acre: 1997

1.66 DUs/acre

Density of average new acre developed: 1987–1997

0.99 DUs/acre

New development as a share of 1997 average

60 percent as dense as the average

Based on census data, 1990–2000 (gross acres)

 

Density of average acre (Method A)

 

Density of average acre: 1990

1.13 DUs/acre

Density of average acre: 2000

1.11 DUs/acre

Density of average new acre developed: 1990–2000

0.96 DUs/acre

New development as a share of 2000 average

86 percent as dense as the average

Density of average DU (Method B)

 

Density of average DU: 1990

3.02 DUs/acre

Density of average DU: 2000

2.89 DUs/acre

Density of average new DU developed: 1900–2000

1.93 DUs/acre

New development as a share of 2000 average

67 percent as dense as the average

Note: See Tables C-2 and C-3 for details.

TABLE C-2 Change in Dwelling Units by Urban Acre, 1987–1997, Based on NRI

Year

Number of Dwelling Units (DUs)

Urban Acresa

DUs/Urban Acre

1987

75,307,500

40,543,000

1.86

1997

87,412,760

52,776,100

1.66

Density of average acre, new and existing development, 1997: 1.66

Density of average new acre developed, 1987–1997: 0.99b

Density of average new acre developed as a share of new and existing development in 2000: 60 percentc

aNote that urban acres correspond to counties within metropolitan statistical areas and consolidated metropolitan statistical areas as defined by the U.S. census prior to 2000. Thus, they are consistently defined for the 1987–1997 time frame used here.

b0.99 = (87,412,760 − 75,307,500)/(52,776,100 − 40,543,000).

c(0.99/1.66) × 100 = 60 percent.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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ÿþ

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

Density of Average New Dwelling Unit Developed (Method B)

Range— DU/gross acre

Average DU/gross acre

Persons/ square miled

Total Number of Dwelling Units

Percent of Total

Change 1990–2000

1990

2000

Change: 1990–2000

1990

2000

Percentage

<1

0.6

960

37,667,907

43,717,293

6,049,386

40.1

41.0

47.5

1–1.9

1.4

2,240

14,071,224

15,680,741

2,609,517

15.0

15.6

20.5

2–2.9

2.4

3,840

11,311,617

13,186,390

1,874,773

12.0

12.4

14.7

3–3.9

3.5

5,600

8,612,696

9,588,164

975,468

9.2

9.0

7.7

4–7.9

5.5

8,800

12,632,087

13,438,555

806,468

13.5

12.6

6.3

8+

12.0

19,200

9,585,988

9,993,097

407,109

10.2

9.4

3.2

Total

93,881,519

106,604,240

12,722,721

100.0

100.0

100.0

Density of average DU, new and existing development, 2000

Density of average new DU developed: 1990–2000

DU/gross acre: 2.89e

DU/gross acre: 1.93e

Persons/square mile: 4,624f

Persons/square mile: 3,083f

Density of average new DU developed as a share of new and existing development in 2000: 67 percentg

aThe number of acres is estimated by dividing the number of DUs in each density category (see above) by the aggregate density of that category. For example, in 2000, the average number of dwelling units <1 DU/acre was 43,717,293, and the aggregate density for that category was 0.6. Thus, the DUs in that category required a total of 72,862,155 gross acres.

bDU/average acre is calculated by dividing the total number of DUs in 2000 (see above) by the total number of acres in 2000.

cPersons/square mile is calculated by multiplying the average DU/gross acre by 2.5 persons/household by 640.

dAssumes 2.5 persons/household. There are 640 acres in 1 square mile.

eThe average DU/gross acre is calculated by multiplying the percentage of DUs by the average DU/gross acre for each category and dividing the total by 100.

fThe average number of persons/square mile is calculated by multiplying the average DU/gross acre by 2.5 persons/household by 640.

g(1.93/2.89) × 100 = 67 percent.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

(calculated by using Method A) was 1.66 DUs per urban acre in 1997 (when NRI data are used) but only 1.11 DUs per gross acre in 2000 (when census data are used). Densities are lower when calculated with Method A than with Method B because dense developments account for a smaller proportion of developed acres than of DUs. On the basis of the 2000 census data, for example, the average acre in a metropolitan or micropolitan area was developed to a density of 1.1 DUs per acre (Method A), while the average DU was located in a tract with a density of 2.89 DUs per acre (Method B). Densities are lower using Method A because in 2000 only 24 percent of the acres were in census tracts that were developed to densities of at least 1 DU per acre, while 59 percent of the DUs were in tracts with at least 1 DU per acre.

DEVELOPMENT TRENDS

The trends in density are similar regardless of which data source or method of calculating density one uses. Over the decade of the 1990s, average densities declined because the average density of new development was lower than the average density of the existing development.6 On the basis of the NRI data, the density of the average acre (Method A) fell from 1.86 DUs per acre in 1987 to 1.66 DUs per acre in 1997 because the new development between 1987 and 1997 was built to a density of only 0.99 DUs per acre. On the basis of the census data, the density of the average acre fell less sharply, from 1.13 DUs per acre in 1990 to 1.11 in 2000 because the density of new development was 0.96 DUs per acre, only slightly less than the density of the existing development in 1990. The more detailed census data reveal some of the patterns behind these averages. The density of the average acre (Method A) declined between 1990 and 2000 because more than three-quarters (76.5 percent) of acres developed in this period were in tracts with

6

The NRI data measure changes from 1987 to 1997, the most recent year for which such detailed data are available.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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below 1 DU per gross acre (see Table C-3). The density of the average DU (Method B) declined slightly less sharply because only 47.5 percent of the new DUs developed during this period were built in census tracts with below 1 DU per gross acre and another 20 percent in tracts with between 1 and 2 DUs per gross acre. Less than one-third (31.9 percent) of new DUs were built in tracts having 2 or more DUs per gross acre (see Table C-3). In short, under either method, the majority of new development occurred in the lowest density categories.

COMPARISON OF TRENDS AND SCENARIOS

The committee’s two scenarios assume that densities are doubled for 25 percent (Scenario 1) and 75 percent (Scenario 2) of all new residential housing built and replaced from 2000 to 2030 and to 2050. How difficult would it be to achieve these densities? The committee addressed this question by using the density data described previously to project current trends toward low-density development forward to 2050 and then comparing the higher densities assumed in the two scenarios with the baseline. Although the same calculations were conducted for 2030 as well, only the 2050 projections are presented here because the durability of the housing stock means that decades are required for the projected changes in development patterns to take hold.

The committee recognizes that the location of more compact development within a metropolitan area is likely to affect the reduction in vehicle miles traveled (VMT). In the committee’s simple projections reported in Chapter 5, however, the VMT estimates are sensitive only to the average density of urban areas, not the spatial distribution of that density within urban areas. Thus while the committee uses the doubling of density in 25 or 75 percent of new development as a shorthand for describing its two scenarios, there are many ways in which an equivalent change in average urban density could be achieved. The committee assumed that much of the new development would occur at the urban fringe where developable land is available. But

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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some amount of infill is also likely, particularly in Scenario 2, which assumes that a large share (75 percent) of new development is at higher densities.

Calculations Using NRI Data

On the basis of the data from the NRI, which pick up lower densities at the urban fringe and leapfrogged development, if all new and replacement housing were to be built at the current average density of new development (the base case scenario)—0.99 DUs per urban acre— the average density of the housing stock would fall from 1.66 DUs per urban acre in 2000 to 1.29 to 1.39 DUs per urban acre by 2050 (see Table C-4). If 25 percent were developed at double the density (1.98 DUs per urban acre) (Scenario 1), average densities would decline from 1.66 to only between 1.43 and 1.49 DUs per urban acre, a 7 to 11 percent increase from the 2050 baseline. Average densities still decline in this scenario because even with the doubling of density for 25 percent of new development, the average density of new development from 2000 to 2050 (1.24 DUs per urban acre)7 is still below the average density of existing development in 2000 (1.66 DUs per urban acre).

In the upper-bound scenario (Scenario 2), which assumes 75 percent of new and replacement housing is built at these higher densities, the average density in 2050 would actually increase from the 2000 average of 1.66 DUs per urban acre to between 1.69 and 1.7 DUs per urban acre, a 22 to 32 percent increase from the 2050 baseline (see Table C-4). Average densities would be higher in 2050 than they were in 2000 because the doubling of density for 75 percent of new development would increase the average density of new development (1.73 DUs per urban acre)8 above the average density of existing development in 2000 (1.66 DUs per urban acre).

7

(1.98 × 0.25) + (0.99 × 0.75) = 1.24.

8

(1.98 × 0.75) + (0.99 × 0.25) = 1.73.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
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TABLE C-4 Changes in Density to Meet Targets of Committee Scenarios by 2050

NRI Data

Census Data (Method B Only)a

 

Number of Units (millions)

Density Level (DU/urban acre)

Percent Change from Base Case

 

Number of Units (millions)

Density Level (DU/gross acre)

Percent Change from Base Case

Base case

 

 

 

Base case

 

 

 

Original housing stock in 2050

86.64–90.35

1.66

 

Original housing stock in 2050

86.64–90.35

2.9

 

New and replacement units

62.44105.37

0.99

 

New and replacement units

62.44105.37

1.9

 

Total in 2050

152.79–192.01b

1.29–1.39

Total in 2050

152.79–192.01b

2.35–2.49

Scenario 1 (25 percent)

 

 

 

Scenario 1 (25 percent)

 

 

 

Original housing stock in 2050

86.64–90.35

1.66

 

Original housing stock in 2050

86.64–90.35

2.9

 

New, more compact

15.61–26.34

1.98c

 

New, more compact

15.61–26.34

3.8c

 

New, not more compact

46.8379.02

0.99

 

New, not more compact

46.8379.02

1.9

 

Total in 2050

152.79–192.01

1.43–1.49

7.2–10.8

Total in 2050

152.79–192.01

2.61–2.69

8–11.1

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

NRI Data

Census Data (Method B Only)a

 

Number of Units (millions)

Density Level (DU/urban acre)

Percent Change from Base Case

 

Number of Units (millions)

Density Level (DU/gross acre)

Percent Change from Base Case

Scenario 2 (75 percent)

 

 

 

Scenario 2 (75 percent)

 

 

 

Original housing stock in 2050

86.64–90.35

1.66

 

Original housing stock in 2050

86.64–90.35

2.9

 

New, more compact

46.83–79.02

1.98c

 

New, more compact

46.83–79.02

3.8c

 

New, not more compact

15.6126.34

0.99

 

New, not more compact

15.6126.34

1.9

 

Total in 2050

152.79–192.01

1.69–1.70

21.6–31.8

Total in 2050

152.79–192.01

3.07–3.13

23.3–33.2

Note: Scenario 1 assumes 25 percent of all new and replacement housing is built more compactly. Scenario 2 assumes 75 percent of all new and replacement housing is built more compactly.

aOnly Method B was selected to illustrate the scenario results using census data because the population-weighted density measure better reflects how the average resident experiences density (see Box C-2).

bAssumes the same range of housing units projected in Chapter 4, Table 4-3.

cAssumes density of new development is doubled (0.99 × 2 = 1.98 for NRI data; 1.9 × 2 = 3.8 for census data).

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

Calculations Using Census Data

According to the census data, if all new and replacement housing were to be built at the current average density of new development (the base case scenario)—1.9 DUs per gross acre—the average density of the housing stock would fall from 2.9 DUs per gross acre in 2000 to 2.35 to 2.49 DUs per gross acre by 2050 (see Table C-4). If 25 percent were developed at double the density (3.8 DUs per gross acre) (Scenario 1), the average density of new development (2.38 DUs per gross acre)9 would still be slightly below the density of the development that existed in 2000. As a result, average densities would fall slightly from 2.9 DUs per gross acre in 2000 to between 2.61 and 2.69 DUs per gross acre in 2050, an 8 to 11 percent increase from the 2050 baseline.

In the upper-bound scenario (Scenario 2), which assumes 75 percent of new and replacement housing is built at these higher densities, new development is denser (3.33 DUs per acre)10 than existing development, so average density increases from 2.9 DUs per gross acre in 2000 to between 3.07 and 3.13 DUs per gross acre, a 23 to 33 percent increase from the 2050 baseline. As with the NRI data, the census data show that doubling density in 25 percent of new development is equivalent to assuming that the average new development is slightly less dense than the average existing development. In contrast, doubling density in 75 percent of new development is comparable with assuming that the average new development is moderately more dense than the average existing development.

Seen in this way, both scenarios are substantial departures from existing patterns in that new development has historically been significantly less dense than existing development. In some metropolitan areas, however, even a growing area such as Phoenix, densities for new development are similar to or slightly higher than densities for existing development, so the 25 percent doubling scenario is possible. But as

9

(3.8 × 0.25) + (1.9 × 0.75) = 2.38.

10

(3.8 × 0.75) + (1.9 × 0.25) = 3.33.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

discussed in Chapter 5, Phoenix has unusual characteristics (e.g., bounded by desert, limits on development), so that the 25 percent doubling scenario for all metropolitan areas would require substantial policy intervention. A 75 percent doubling scenario would require even greater policy intervention and public acceptance because it assumes that new developments are denser than existing developments, and, if compared with a continuing trend of new, low-density development to 2050, significantly more dense than the 2050 baseline.

The more detailed census data allow a test of the plausibility of the two scenarios by showing how much the different density categories would have to change to reach the target densities implicit in the scenarios. Meeting the target of Scenario 1—doubling density in 25 percent of new development—could be achieved (or nearly so) in a variety of ways. For example, one way would be to eliminate half of all new development in tracts in the lowest density category (less than 1 DU per gross acre), substantially reducing development in many urban fringe areas where lots are very large (see Strategy B in Table C-5). Alternatively, one could follow an infill strategy and double the densities in tracts with an average of 4 or more DUs per acre (see Strategy C in Table C-5).

Reaching the target of Scenario 2—doubling density in 75 percent of new development—would require much more drastic measures, for example, eliminating all new DUs in tracts where development currently averages less than 1 DU per acre (see Strategy A in Table C-5) or doubling the density of tracts with an average of 3 or more DUs per acre, which would be a much more aggressive infill scenario (see Strategy D in Table C-5). Note that the comparisons made here are based on measures of the density of the average DU (Method B).

Using an alternative way of measuring density (Method A)—by the density of the average developed acre rather than the density of the average DU—changes somewhat the ease (or difficulty) of the various ways of achieving the target densities. For example, infill scenarios are less effective. Even the aggressive infill scenario—doubling the density of tracts with 3 or more DUs per acre—does not achieve either the

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

TABLE C-5 Alternative Strategies for Achieving Higher Densities for New Development Implied in Committee Scenario Targets

Baseline

DUs/Acre (range)

Average DU/Acre

Change in DUs, 1990–2000

Percent of Total

Change in Acres, 1990–2000

Percent of Total

<1

0.6

6,049,386

47.5

10,082,310

76.5

1–1.9

1.4

2,609,517

20.5

1,863,941

14.1

2–2.9

2.4

1,874,773

14.7

781,155

5.9

3–3.9

3.5

975,468

7.7

278,705

2.1

4–7.9

5.5

806,468

6.3

146,631

1.1

8+

12.0

407,109

3.2

33,926

0.3

Total

12,722,721

100.0

13,186,668

100.0

Strategy A: Eliminate All New DUs in Lowest-Density Tracts, Redistribute Proportionately

DUs/Acre (range)

Change in DUs

Percent of Total

Change in Acres

Percent of Total

<1

0

0

0

0

1–1.9

4,975,047

39

3,553,605

60

2–2.9

3,574,257

28

1,489,274

25

3–3.9

1,859,731

15

531,352

9

4–7.9

1,537,532

12

279,551

5

8+

776,154

6

64,679

1

Total

12,722,721

100

5,918,461

100

 

 

 

DUs/Acre

Persons/Acrea

Persons/Square Mileb

Density of average acre (Method A)

 

 

 

Baseline

0.96

2.40

1,536

Strategy A

2.15

5.37

3,440

Percent change

129

129

129

25 percent higher density target

1.2

3.0

1,920

75 percent higher density target

1.68

4.2

2,688

Density of average DU (Method B)

 

 

 

Baseline

1.93

4.82

3,085

Strategy A

3.13

7.82

5,008

Percent change

62

62

62

25 percent higher density target

2.41

6.02

3,856

75 percent higher density target

3.38

8.43

5,399

Page 228
Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

Strategy B: Reduce by One-Half New DUs in Lowest-Density Tracts, Redistribute Proportionately

DUs/Acre (range)

Change in DUs

Percent of Total

Change in Acres

Percent of Total

<1

3,024,693

24

5,042,155

53

1–1.9

3,792,282

30

2,708,773

28

2–2.9

2,724,515

21

1,135,215

12

3–3.9

1,417,599

11

405,028

4

4–7.9

1,172,000

9

213,091

2

8+

591,631

5

49,303

1

Total

12,722,721

100

9,552,564

100

 

DUs/Acre

Persons/Acrea

Persons/Square Mileb

Density of average acre (Method A)

 

 

 

Baseline

0.96

2.40

1,536

Strategy B

1.33

3.33

2,128

Percent change

38

38

38

25 percent higher density target

1.2

3.0

1,920

75 percent higher density target

1.68

4.2

2,688

Density of average DU (Method B)

 

 

 

Baseline

1.93

4.82

3,085

Strategy B

2.39

5.97

3,721

Percent change

24

24

24

25 percent higher density target

2.41

6.02

3,856

75 percent higher density target

3.38

8.43

5,399

Strategy C: Double New DUs in Highest-Density Tracts (>4 DU/Acre), Redistribute Proportionately

DUs/Acre (range)

Change in DUs

Percent of Total

Change in Acres

Percent of Total

<1

5,411,511

43

9,019,185

75

1–1.9

2,334,358

18

1,667,398

14

2–2.9

1,667,088

13

698,787

6

3–3.9

872,610

7

249,317

2

4–7.9

1,612,936

13

293,261

2

8+

814,218

6

67,852

1

Total

12,722,721

100

11,995,800

100

Page 229
Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

 

DUs/Acre

Persons/Acrea

Persons/Square Mileb

Density of average acre (Method A)

 

 

 

Baseline

0.96

2.40

1,536

Strategy C

1.06

2.65

1,696

Percent change

10

10

10

25 percent higher density target

1.2

3.0

1,920

75 percent higher density target

1.68

4.2

2,688

Density of average DU (Method B)

 

 

 

Baseline

1.93

4.82

3,085

Strategy C

2.28

5.70

3,648

Percent change

18

18

18

25 percent higher density target

2.41

6.02

3,856

75 percent higher density target

3.38

8.43

5,399

Strategy D: Double DUs in Tracts Averaging >3 DUs/Acre, Redistribute Proportionately

DUs/Acre (range)

Change in DUs

Percent of Total

Change in Acres

Percent of Total

<1

4,792,239

38

7,987,065

73

1–1.9

2,067,223

16

1,476,588

13

2–2.9

1,485,169

12

618,820

6

3–3.9

1,950,936

15

557,410

5

4–7.9

1,612,936

13

293,261

3

8+

814,218

6

67,852

1

Total

12,722,721

100

11, 000,996

100

 

 

DUs/Acre

Persons/Acrea

Persons/Square Mileb

Density of average acre (Method A)

 

 

 

Baseline

0.96

2.40

1,536

Strategy D

1.16

2.90

1,856

Percent change

21

21

21

25 percent higher density target

1.2

3.0

1,920

75 percent higher density target

1.68

4.2

2,688

Density of average DU (Method B)

 

 

 

Baseline

1.93

4.82

3,085

Strategy D

2.51

6.27

4,016

Percent change

30

30

30

25 percent higher density target

2.41

6.02

3,856

75 percent higher density target

3.38

8.43

5,399

aPersons/acre equals number of DUs/acre times 2.5 persons/household.

bPersons/square mile equals persons/acre multiplied by 640 acres.

Page 230
Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

25 percent or the 75 percent target because doing so prevents much less land from being used for development than curtailing development in the lowest-density tracts where the average acres per DU are largest. Pursuing the latter strategy—eliminating all new development in tracts with densities below 1 DU per acre—results in achieving not only the 25 percent but also the 75 percent target. The reason is the large amount of acreage in the below-1-acre category (see the baseline in Table C-5). Because such large lots represented approximately 75 percent of the residential land developed between 1990 and 2000, this strategy would require an almost complete reversal of recent development patterns. Given the large amount of land available for development and lax land use policies in more rural counties, achieving such a goal would require extraordinary changes in land use policy and market trends.

The analyses provided here suggest that both the 25 and the 75 percent targets represent a significant departure from recent trends, which have involved lower densities than the average for new development over the decade of the 1990s and for decades before that. Doubling the density of 25 percent of new development by 2050 will not raise densities above the current average and will raise them only about 7 to 11 percent above the 2050 baseline. In addition, precedents for such changes in density can be found even in growing areas such as Phoenix. Nevertheless, Phoenix is not typical of many growing metropolitan areas, and meeting the 25 percent target will require a trend change. Doubling the density of 75 percent of new development by 2050 will require densities above those of existing developments, significantly above (approximately 20 to 33 percent) the 2050 baseline.

The committee disagreed about the feasibility of doubling the density of 75 percent of new development, even by 2050. Those members who thought it was possible questioned whether densities will keep declining. The combination of macroeconomic trends—likely higher energy prices and carbon taxes—in combination with growing public support for strategic infill, investments in transit, and higher densities

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×

along rail corridors, could result in considerably higher densities by 2050. Other members believed that the substantial infill or curbing of large-lot development at the urban fringe, or both, implied in achieving the targets in Scenario 2 would require such a significant departure from current housing trends, land use policies of jurisdictions on the urban fringe, and public preferences that they would be unrealistic absent a strong state or regional role in growth management.

REFERENCES

Abbreviations

NRCS Natural Resources Conservation Service

OMB Office of Management and Budget

Downs, A. 2004. Still Stuck in Traffic: Coping with Peak-Hour Traffic Congestion. Brookings Institution, Washington, D.C.

NRCS. 2002. National Resources Inventory, 2002 and 2003 Annual NRI, Glossary of Key Terms. U.S. Department of Agriculture, Washington, D.C. www.nrcs.usda.gov/technical/NRI/2002/glossary.html. Accessed Aug. 28, 2008.

NRCS. 2003. National Resources Inventory, 2001 Annual NRI, Urbanization and Development of Rural Land. U.S. Department of Agriculture, Washington, D.C., July. www.nrcs.usda.gov/technical/NRI/2001/urban.pdf.

OMB. 2000. Standards for Defining Metropolitan and Micropolitan Statistical Areas. Federal Register, Vol. 65, No. 249, pp. 82228–82238.

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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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Suggested Citation:"C Analysis of Density Assumptions and Feasibility of Committee Scenarios." Transportation Research Board and National Research Council. 2009. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. doi: 10.17226/12747.
×
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TRB Special Report 298: Driving and the Built Environment: Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions examines the relationship between land development patterns and vehicle miles traveled (VMT) in the United States to assess whether petroleum use, and by extension greenhouse gas (GHG) emissions, could be reduced by changes in the design of development patterns. The report estimates the contributions that changes in residential and mixed-use development patterns and transit investments could make in reducing VMT by 2030 and 2050, and the impact this could have in meeting future transportation-related GHG reduction goals.

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