Land Use Impacts On Travel: Current State of Knowledge
As discussed in my previous column, An Inaccurate Attack On Smart Growth, the National Association of Home Builders (NAHB) sponsored a research program intended to raise doubts about smart growth's ability to reduce vehicle travel, conserve energy and reduce pollution emissions. They claim that, "The existing body of research demonstrates no clear link between residential land use and GHG emissions and leaves tremendous uncertainty as to the interplay of these factors," and "The assumption of a causal connection between density and GHG emissions is based on prevailing beliefs within the planning community and not on verifiable scientific research or analysis" (NAHB 2010).
I believe that this is incorrect and severely misrepresents the current state of knowledge concerning the relationships between land use factors, travel activity, energy consumption and pollution emissions. This column provides a brief review of the literature on this subject.
The disciplines of geography, planning, transport modeling and urban economics all recognize that various land use factors can affect travel activity (the amount and type of travel that occurs in an area), as summarized in the following table. For detailed reviews of this research see Ewing and Cervero's recent summary article, the California Air Resources Board (CARB) ongoing research program, and my report Land Use Impacts on Transport, which is regularly updated as new information becomes available.
An extensive and growing body of research investigates these relationships and the role that smart growth policies can play in achieving various planning objectives including infrastructure cost savings, consumer savings, improved public safety and health, energy conservation and emission reductions. Studies by major professional organizations and agencies, including the American Association of State Highway and Transportation Officials (AASHTO 2009), the Transportation Research Board (TRB 2009), the Urban Land Institute (ULI 2010) and the U.S. Department of Transportation (USDOT 2010) all conclude that smart growth land use policies can help achieve various planning objectives. Based on this research many jurisdictions are adopting smart growth policies.
Research on the relationships between land use and transportation is constrained by data availability. Ideally, researchers want detailed demographic (age, income, family type, etc.), land use (density, mix, connectivity, walkability, parking supply, etc.) and travel activity (per capita walking, cycling, public transit and automobile travel) data disaggregated at a fine scale for many different areas. Until recently, such data was seldom available so most early studies considered a limited number of factors and limited geographic areas. Since land use factors are interrelated (urban areas tend to have higher density, mix, connectivity, transit service quality, walkability, parking prices, etc.), and density is generally the easiest factor to calculate, it was common to use density as a surrogate for urbanization, often called "compact development."
The results of early studies varied widely depending on their analysis methods and geographic locations. Some indicated that by itself a particular land use factor such as density or connectivity had little or no impact on travel activity. In some cases, critics intentionally confuse the issue by suggesting that, because density itself (holding all other factors constant) has only modest impacts on travel activity, smart growth cannot achieve significant emission reductions.
During the last decade an increasing amount of geocoded data has become available, allowing more integrated and comprehensive research (Bento, et al. 2005; Brownstone and Golob 2009; Fang 2008; Frank, et al. 2011; Heres-Del-Valle and Niemeier 2011). For example, it is now possible to assemble a dataset that includes population and employment density, land use mix, road connectivity, transit service accessibility, fuel price, and even indicators of walkability, as well as population demographic and economic activity data. These more technically sophisticated studies tend to indicate relatively strong impacts of land use on travel activity, confirming the potential benefits of smart growth policies. In fact, research sponsored by the NAHB found that land use factors and transit service quality significantly affect travel activity (Liu 2007).
Smart growth critics have seized on the sometimes contradictory findings of early research and ignore the more recent research when they argue that results are uncertain and smart growth policies are ineffective. This misrepresents the real issues. Although there is variation and some degree of uncertainty as to how a particular land use factor will affect travel activity in a particular situation, good research indicates that integrated smart growth programs that include a variety of complementary stategies can provide significant impacts and benefits.
One of the most absurd claims by smart growth critics is that, "At a theoretical level there is no obvious connection between compact development and mode choice" (Fruits 2011). These relationships reflect the economics of accessibility. Density, mix, conectivity and centricity affect the travel distances, and therefore the time and money costs, of reaching destinations. Factors such as parking supply and price affect the financial costs of driving. Walking and cycling conditions, and transit service quality affect the relative attractiveness of different travel modes. An important factor in people's travel decisions is that most communities are relatively automobile-dependent (it is difficult to live without an automobile), and most vehicle costs are fixed, so once residents purchase a car they have little incentive to use alternatives. As a result, pricing reforms that convert fixed vehicle costs into variable costs (e.g., distance-based insurance and user-paid parking), and more multi-modal community design that allows households to reduce their vehicle ownership, can have large effects on travel activity, reducing per capita automobile travel and increasing use of alternative modes.
Critics significantly understate smart growth's potential impacts and benefits. Actual travel impacts are probably four to eight times greater than NAHB claims (doubling all land use factors typically reduces affected vehicle travel 20-40%, compared with the 5% they cite), and total benefits are far greater due to additional economic, social and environmental co-benefits. This is not to deny that smart growth can also impose incremental costs, such as increased infrastructure expenses (for curbs and sidewalks) and smaller size lots. However, these incremental costs should be compared with total benefits. In a future column I will discuss current research on these benefits.
References and Information Resources
AASHTO (2009), Real Transportation Solutions for Greenhouse Gas Emissions Reductions, American Association of State Highway and Transportation Officials (www.transportation.org); at www.transportation1.org/RealSolutions/index.html.
Antonio M. Bento, Maureen L. Cropper, Ahmed Mushfiq Mobarak and Katja Vinha (2003), The Impact of Urban Spatial Structure on Travel Demand in the United States, World Bank Group Working Paper 2007, World Bank (http://papers.ssrn.com/sol3/papers.cfm?abstract_id=636369); published in The Review of Economics and Statistics (http://mitpress.mit.edu), Vol. 87, Issue 3 - August 2005, pp. 466 – 478.
David Brownstone (2008), Key Relationships Between the Built Environment and VMT, for the Committee on the Relationships Among Development Patterns, Vehicle Miles Traveled, and Energy Consumption; for Special Report 298, Driving And The Built Environment: The Effects Of Compact Development On Motorized Travel, Energy Use, And CO2 Emissions, Transportation Research Board (www.trb.org); at http://onlinepubs.trb.org/Onlinepubs/sr/sr298brownstone.pdf.
CARB (2010), Research on Impacts of Transportation and Land Use-Related Policies, California Air Resources Board (http://arb.ca.gov/cc/sb375/policies/policies.htm).
Eric Eidlin (2010), "What Density Doesn't Tell Us About Sprawl," Access 37, University of California Transportation Center, pp. 2-9; at www.uctc.net/access/37/access37_sprawl.shtml.
Reid Ewing and Robert Cervero (2010), "Travel and the Built Environment: A Meta-Analysis," Journal of the American Planning Association, Vol. 76, No. 3, Summer, pp. 265-294; at www.climateplanca.org/Travel_Built_Environ.pdf.
Hao Audrey Fang (2008), "A Discrete–Continuous Model Of Households' Vehicle Choice And Usage, With An Application To The Effects Of Residential Density," Transportation Research Part B, Vol. 42, pp. 736–758; summary at www.sciencedirect.com/science/article/pii/S019126150800012X.
Lawrence D. Frank , et al. (2011), An Assessment of Urban Form and Pedestrian and Transit Improvements as an Integrated GHG Reduction Strategy, Washington State Department of Transportation (www.wsdot.wa.gov); at www.wsdot.wa.gov/research/reports/fullreports/765.1.pdf.
Eric Fruits (2011), "Compact Development And Greenhouse Gas Emissions: A Review Of Recent Research," Center for Real Estate Quarterly Journal, Vol. 5, No. 1. Winter; at www.pdx.edu/sites/www.pdx.edu.realestate/files/media_assets/quarterly_report/march_2011/01%20Fruits%20Quarterly%202011-02.pdf.
David Heres-Del-Valle and Deb Niemeier (2011), "CO2 Emissions: Are Land-Use Changes Enough For California To Reduce VMT? Specification Of A Two-Part Model With Instrumental Variables," Transportation Research Part B, Vol. 45, pp. 150–161.
Todd Litman (2011a), "Can Smart Growth Policies Conserve Energy and Reduce Emissions?" Portland State University's Center for Real Estate Quarterly (www.pdx.edu/realestate/research_quarterly.html), Vol. 5, No. 2, Spring, pp. 21-30; at www.vtpi.org/REQJ.pdf.
Todd Litman (2011b), An Inaccurate Attack On Smart Growth, Planetizen (www.planetizen.com/node/49772).
Helen Fei Liu (2007), Vehicle CO2 Emissions and the Compactness of Residential Development, HousingEconomics.com; at www.nahb.org/generic.aspx?sectionID=734&genericContentID=86266&channelID=311.
NAHB (2010), Climate Change, Density and Development: Better Understanding the Effects of Our Choices, National Association of Home Builders (www.nahb.org); at www.nahb.org/fileUpload_details.aspx?contentTypeID=3&contentID=151627&subContentID=319667.
Caroline Rodier (2009) A Review of the International Modeling Literature: Transit, Land Use, and Auto Pricing Strategies to Reduce Vehicle Miles Traveled and Greenhouse Gas Emissions, Transportation Sustainability Research Center and the Institute of Transportation Studies (www.its.ucdavis.edu); at http://pubs.its.ucdavis.edu/publication_detail.php?id=1241.
Robert Steuteville (2011), Does Smart Growth Reduce Carbon Emissions? Bet The House On It, New Urban Network (www.newurbannetwork.com); at http://newurbannetwork.com/news-opinion/blogs/robert-steuteville/14788/does-smart-growth-reduce-carbon-emissions-bet-house-it.
TRB (2009), Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions, Special Report 298, Transportation Research Board (www.trb.org); at http://onlinepubs.trb.org/Onlinepubs/sr/sr298.pdf.
ULI (2010), Land Use and Driving: The Role Compact Development Can Play in Reducing Greenhouse Gas Emissions, Urban Land Institute (www.uli.org); at www.uli.org/ResearchAndPublications/PolicyPracticePriorityAreas/Infrastructure.aspx.
USDOT (2010), Transportation's Role in Reducing U.S. Greenhouse Gas Emissions: Report to Congress, U.S. Department of Transportation (www.dot.gov), at http://ntl.bts.gov/lib/32000/32700/32779/DOT_Climate_Change_Report_-_April_2010_-_Volume_1_and_2.pdf.