For climate stabilization, the United States (US) will need to cut its transportation carbon dioxide (CO2) emissions by 60%–80% by 2050. As we cannot accomplish this reduction through vehicle and fuel technology alone, the environmental and transportation communities are focusing on strategies to reduce growth in vehicle miles traveled (VMT), reversing recent trends. The majority of recent VMT growth is due, not to population growth, but to the effects of urban environment, such as increased auto ownership, longer trips, and driving alone. The US Department of Energy forecasts that driving will continue to increase at almost threefold the growth in population. Even under the most stringent vehicle and fuel standards, transportation-related CO2 emissions will be 40% above the target level. In response, climate-change legislation has been passed in California and is pending in other states and in the US Congress that places strict new requirements on mandated environmental impact documentation. One limitation on compliance has been the lack of a unified set of scientific information on the underlying relationships between development form and VMT generation. This article distills and reconciles various forms of prior research on the subject, producing a unified quantitative understanding of the mechanisms that relate urban development forms with VMT and CO2. The findings will help improve the insightfulness and accuracy of the next generation of environmental documents. The article provides results of research and planning studies from throughout the US that indicate the degree to which developments with higher densities, mix of uses, accessible destinations, and interconnected streets reduce vehicle trips and VMT. Sources include regional blueprint studies, as well as project-specific studies such as Atlanta's Atlantic Station development, whose predicted trip reduction was found to be so compelling that the development was deemed a transportation control measure and air quality benefit by the Environmental Protection Agency and Federal Highway Administration. The article identifies the emerging practices in transportation impact analysis for assessing the degree to which the characteristics of development can reduce VMT and carbon impacts. It presents the empirical evidence of how vehicle travel is affected by density, diversity, walkability, regional accessibility, and distance from transit. It also reviews tools that transportation planners and analysts have available to capture these effects, including simple elasticities and new trip generation rates under consideration by the Institute of Transportation Engineers.
Environmental Practice 11:196–208 (2009)