The Science of Reducing Emissions: Which Transportation and Land-Use Strategies Show the Most Promise?

As the saying goes, “Put your money where your mouth is.”

Well, California is doing just that. Thanks to the Cap-and-Trade Program, the state has $832 million to spend in fiscal year 2014-15 on projects that will help chip away at the ambitious target of reducing greenhouse gas (GHG) emissions—and even more is expected in future years. A portion of these funds ($130 million) will be passed along to California communities, in part to support projects that help people replace car trips with low-carbon transportation options (e.g., walking, bicycling, and transit) and in part to preserve agricultural lands.

This opportunity has fueled extensive conversations about the best ways to reduce GHG emissions while addressing other important social, environmental, and economic priorities in regions across California. Many people are asking: Which strategies will provide the greatest GHG bang for the buck? In answering this question, looking to the research is a good place to start.

With funding from the California Air Resources Board (ARB) and a directive to make sense of the existing research, Dr. Marlon Boarnet of the University of Southern California and I worked with a team of graduate students and post-doctoral researchers.  We completed an extensive review of the available evidence on 23 strategies for cutting GHG emissions through reduced driving or improved fuel efficiency. We looked at a range of strategies — from car sharing and telecommuting to residential density and network connectivity. We identified relevant studies for each strategy, screened them for quality and relevance to California, and summarized the “effect size” – the percentage reduction in vehicle miles of travel that a given amount of a strategy can produce.

Our review is summarized in short, easy-to-read policy briefs, thus making the research accessible to those who most need to use it. I will be talking more about these strategies on October 7th at an ARB research seminar.

One of the most popular – and widely studied – strategies is densification.  Especially in the major metropolitan areas of the state, local governments are enabling and encouraging higher densities in targeted areas, such as those with good transit access.  Studies show that higher densities do make a difference: an area with twice the density of another may have 5 percent to 12 percent less driving in vehicle miles traveled (VMT).   Regional accessibility, the proximity of a residence to jobs, shopping, and other activities within the region, is also associated with lower VMT, as is land-use mix, the degree to which housing, shopping, and other activities are found together within a small area.

For some of the other new strategies that communities are considering, the evidence is promising. For example, studies of car-sharing services suggest that the average user of the service drives 25 percent to 33 percent less than they would otherwise. Only three studies passed our screening, however, so these estimates are less robust.

We also looked at the evidence on the traditional approach to fixing transportation problems: widening roads. Studies show this strategy tends to increase driving, thereby offsetting some of the expected reductions in congestion and maybe even wiping them out in the long run. Conversely, we found some evidence that decreasing capacity – by removing or closing roads – does not worsen congestion and may even decrease driving, particularly if road space is given over to other modes. These findings were noted in a draft discussion document released recently by the Governor’s Office of Planning and Research that outlines recommended changes to the way transportation impacts are analyzed under the California Environmental Quality Act.

Overall, the evidence is pretty convincing that the transportation and land-use related strategies we examined can be effective in reducing driving and therefore GHG emissions.

That being said, questions remain.  First, it is hard to say exactly how much impact a given strategy will have in any particular place. We simply don’t have enough studies from enough different places to understand how the impact of the strategy might vary. Second, the study participants who were exposed to these strategies are often people who wanted to drive less to begin with – they are, to some degree, “self-selected.” We can’t be sure that the average person would respond to the strategy to the same degree as the study participants.

So what specific steps can a community take to reduce GHG emissions? The evidence is clear on at least one point: No magic bullet exists. Instead, communities must adopt combinations of multiple land-use and transportation strategies if we are to reach state goals of reduced greenhouse gas emissions and more livable communities.

But if we truly want to know what strategies are most effective, we need more research. For this reason, UC Davis is excited to be leading the newly established National Center for Sustainable Transportation, a consortium of leading universities committed to advancing an environmentally sustainable transportation system through cutting-edge research, direct policy engagement, and education of our future leaders. Our university team includes many of the most accomplished researchers in the world in sustainable transportation, with strong expertise in a range of environmental issues, especially GHG mitigation. Our research will focus on three main areas: low-impact travel and supportive land use, low carbon infrastructure and efficient system operations, and zero-emission fuel and vehicle technology. We will be looking at all modes, for both people and goods, from urban to rural settings. Across all our research we will be identifying and evaluating best practices for institutional change . We are committed to achieving meaningful progress, putting research into action by engaging key decision makers at the federal, state, and local level.  Advising us on research priorities and project design is our Leadership Council, comprised of national, state, and regional transportation leaders—all dedicated to the mission of the National Center.

Exciting new developments are occurring in sustainable transportation. Stay tuned and stay connected as we continue to dive deeper into evidence-based strategies for cutting GHG emissions from the transportation sector. To receive updates on our progress and research, be sure to sign up for the National Center’s email list.

Susan Handy is the Director of the National Center for Sustainable Transportation; Associate Director of Education of the UC Davis Institute of Transportation Studies; and Professor and Chair, UC Davis Department of Environmental Science and Policy.

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