Although the CO₂ emissions reductions found to accrue under the two smart growth scenarios were lower than that of fleet hybridization as a stand alone strategy, our findings suggest urban densification holds the potential to approach or even surpass the benefits of hybrid technologies if region-wide densities can be increased beyond the levels modeled herein.

Our results suggest that, all else being equal, a doubling of mean population density throughout the median metropolitan area would have the effect of reducing vehicle CO₂ emissions by about 30% relative to the BAU scenario, while the full dissemination of conventional hybrid technology was found to reduce vehicle CO₂ emissions by 18%.

—Stone et al. (2009)

Stone’s study looked at 11 major metropolitan regions (MSAs, Metropolitan Statistical Areas) in EPA Region V (the Midwest) over a 50-year period and assessed four different scenarios:

• Business-as-Usual Development Scenario (BAU). The BAU scenario assumes future population and land use trends throughout the study region will be consistent with historical change.

• HEV Fleet Scenario (HEV). This scenario assumes that 100% of the study region’s vehicle fleet will be HEVs by 2050—half of which will be grid plug-in HEVs—but that the fleet will otherwise remain unchanged from 2000 in terms of its relative composition of different vehicle size classes.

• Smart-Growth Development Scenario 1 (SG1). This scenario assumes the total population of each MSA in 2050 is the same as under BAU (business-as-usual), but that new population added in each decade is reallocated away from rural to suburban and urban census tracts in response to the widespread adoption of growth management policies, such as urban growth boundaries (UGBs) and transit-oriented development.

• Smart-Growth Development Scenario 2 (SG2). In the SG2 scenario, a larger percentage of new MSA population growth is reallocated from rural to suburban and urban census tracts. Starting in the base year of 2000, all new population growth per MSA is directed to urban and suburban tracts only, with 10% of this growth directed to urban tracts and the remaining 90% directed to suburban census tracts in 2010. The urban allocation then increases by 10 percentage points per decade, rising to 50% of new population growth by 2050. Relative to SG1, SG2 results in higher urban populations and greater mean densities per census tract, as well as a larger number of tracts with densities of 10,000 people per square mile and higher.

The core of the study’s approach is a vehicle activity modeling framework for associating future land use and demographic characteristics, under BAU, SG1, and SG2 scenarios, with household vehicle travel. Using a framework originally developed by researchers at the Oak Ridge National Laboratory (ORNL), the Georgia Tech team estimated VMT (vehicle miles travelled) in response to three census variables: median household income, vehicle ownership, and employment rate, plus the neighborhood type classification.

Even the full hybridization of the national vehicle fleet by 2050 would not meet the CO₂ targets identified though the Kyoto Protocol. According to Stone’s work, meeting this goal in the transportation sector would require a combination of technological improvements and higher density land use patterns in cities.

The results of our analysis suggest that continued dissemination of hybrid-electric engine technology would significantly offset the growth in vehicle emissions above base year levels independent of land use change. Should the full hybridization of the light duty vehicle fleet take place by 2050, it would offset virtually all of the expected growth in CO₂ emissions under the BAU scenario.

An important contrast between the technological and smart growth strategies considered, however, is the implications of each for a wider range of problems associated with urban growth and vehicle use, such as traffic congestion, disinvestment in transit, and low rates of physical activity, as well as the life-cycle energy and pollution associated with the manufacture and disposal of vehicles. In light of the well documented problems associated with vehicle ownership and use in cities, climate change management strategies employing approaches rooted in both land use and technology change hold the potential to achieve the greatest offsets in mobile source emissions while addressing a wider array of problems associated with the growth of the vehicle population. In addition, our findings suggest that CO₂ emission reductions needed to meet the targets established by the Kyoto Protocol (1990 levels or lower) cannot be attained through technology change alone.

—Stone et al. (2009)

The eleven metropolitan regions that were studied include Madison, Wisconsin; Columbus, Ohio; Indianapolis, Indiana; Minneapolis-St. Paul, Minnesota; Cincinnati, Ohio; Grand Rapids, Michigan; Chicago, Illinois; Detroit, Michigan; and Dayton, Ohio. In addition to Stone, Dr. Tracey Holloway, Scot Spak, and Adam Mednick also authored the study.

Resources

• Brian Stone Jr., Adam C. Mednick, Tracey Holloway, and Scott N. Spak (2009) Mobile Source CO₂ Mitigation through Smart Growth Development and Vehicle Fleet Hybridization. Environ. Sci. Technol., Article ASAP • doi: 10.1021/es8021655 • Publication Date (Web): 11 February 2009