Transportation and the uses of land are intimately related. The density and mix of buildings and other features of the built environment shape people's travel needs and habits, which in turn also shape urban form. Areas of higher-density and with mixed commercial and residential buildings-known as compact development-are associated with greater use of modes other than personal vehicles. People walk, cycle, and use other non-motorized transport more because trip distances are typically shorter and are less likely to require travel on major roadways. Transit development and use also tends to be more feasible and desirable in compact areas, where large numbers of people can be served efficiently. When personal vehicles are used in these areas, trips tend to be shorter, and ride sharing is more feasible because there is a greater likelihood that individuals are traveling to and from similar locations (Ewing and Cervero, 2001; Cervero, 2002).
Yet, over much of the second half of the twentieth century, residential density in U.S. urban areas decreased as jobs located in city centers declined and the population living in low densities (e.g., in suburbs) increased. Areas were also often zoned for single use, either commercial or residential. While this has started to change in recent years, Ewing et al. (2008) note, "current conditions reflect the legacy of this era of sprawl."
This raises important questions: Can shifts in land use (e.g., toward higher densities) lead to reductions in fuel consumption and thereby reduce GHG emissions? If so, how can such shifts be brought about, and what levels of reductions can be achieved with different measures?
Recent research suggests that changing land use patterns may indeed be a key element in reducing GHG emissions in transportation. Estimates have been made that doubling residential density across a region could reduce VMT by about 5 to 12% (Committee for the Study on the Relationships Among Development Patterns, Vehicle Miles Traveled, and Energy Consumption, 2009). A recent report examined the potential impact of land use strategies on CO2 emissions found that, by shifting 60% of new residential growth across the United States to compact patterns, CO2 emissions could decline by 7 to 10% from current trends by 2050, or 79 million MTCO2 annually (Ewing et al., 2008).
Numerous efforts are underway to leverage the key relationship between transportation and land use to combat GHG emissions, while also creating more livable, sustainable communities. The Department of Transportation, the Department of Housing and Urban Development (HUD), and the Environmental Protection Agency (EPA) are collaborating closely on the Partnership for Sustainable Communities to "help communities nationwide improve access to affordable housing, increase transportation options, and lower transportation costs while protecting the environment."
At the metropolitan level, the San Diego Association of Governments (SANDAG) is addressing climate change and GHG reduction through its 2050 Regional Transportation Plan (RTP), and its Sustainable Communities Strategy (SCS) that seeks to guide the San Diego region toward a more sustainable future by integrating land use, housing, and transportation planning to create communities that are more sustainable, walkable, transit-oriented, and compact. California's Senate Bill 375 (SB 375), effective in 2009, requires each MPO in California to prepare a SCS as an integrated element of the Regional Transportation Plan. This new element shows how integrated land use and transportation planning can lead to lower GHG emissions from automobiles and light trucks. The 2050 RTP and its SCS seek to guide the San Diego region toward a more sustainable future by focusing housing and job growth in urbanized areas, protecting sensitive habitat and open space, and investing in a transportation network that provides residents and workers with transportation options to reduce greenhouse gas emissions.
For several reasons, this sourcebook treats land use as a backdrop for assessing other strategies, rather than a strategy in and of itself.
For the purposes of this review-to provide DOTs and MPOs with a review of individual, actionable strategies-it is more appropriate to treat land use as a backdrop for assessing other strategies, rather than as a strategy in and of itself; this is the case for two very practical reasons. First, a change in land use (e.g., to higher density) is not itself an action but a consequence of other actions, similar to changes in zoning. Thus, when the sourcebook discusses land use as a strategy, this refers to a large and varied bundle of policies that can be used to effect changes in land use patterns. In addition to zoning, these policies include financial mechanisms encouraging growth in existing neighborhoods, near existing transit stations, and on former industrial sites (called infill and brownfield development); requirements for pedestrian and bicycle access in new developments; and siting new schools with smaller campuses in established neighborhoods.
Second, each of these actions is largely beyond the purview of transportation agencies. The authority to zone land for specific uses and densities generally resides with local governments. For example, even though the evidence indicates that transit ridership may be higher when a station is surrounded by high-density development, the transit agency cannot alone ensure or enable such development: the city that controls the zoning around the station would need to allow it. Currently, transportation investments, which are generally guided by MPOs and state DOTs who control the programming of transportation funds, are often uncoordinated with land use decisions, despite that the two are intimately related. While coordination between different agencies and governments is important, the status quo means that land use strategies cannot be implemented by transportation agencies as, say, traffic signal optimization can be.
These two factors suggest that guidance that focuses entirely on the relationship between land use and transportation, and the ways that transportation and government agencies can work together to effect desired changes, is warranted but beyond the scope of this review. Instead, several other reports have attempted to do just this:
Practical concerns for this review notwithstanding, there are a number of other reasons that policies to change land use must be assessed differently from strategies like traffic signal optimization, transit incentives, or eco-driving campaigns. Land use patterns are affected by other, potentially much larger forces, such as economic conditions, the social and political climate, and people's preferences in lifestyle. Moreover, the effects of these policies typically emerge slowly, sometimes over decades, and in conjunction with the effects of these other driving factors. Correspondingly, the effects on transportation may take decades to emerge and are also shaped by broader social, economic, and political trends. For all of these reasons, it is difficult to predict the effects of these actions on GHG emissions. In addition to and partly for these reasons, most of the studies in this area focus on the relationship between existing land use patterns and transportation patterns, not on the effects of particular actions aimed at changing land use over a period of time.
Unlike most other strategies considered in this review, land use patterns affect much more than transportation: they can affect the balance of power between local and regional governments, the provision of utilities, housing markets and economic growth, and personal lifestyles. The effects on these segments-including on their GHG emissions and costs-should also be taken into account to understand the full effect of these actions.
These factors suggest that there are important research opportunities in assessing the effects of different actions on land use and subsequently on transportation, in developing new modeling and analyses methods that yield estimates of GHG effects, in evaluating the effects of these actions on other sectors, and in developing methods by which transportation agencies and local governments can work together on these issues.
In sum, the nexus between land use and transportation is important and may be critical to reducing GHG emissions. However, this sourcebook presents land use as a backdrop against which other strategies should be assessed, rather than as its own strategy. Where land use is known to play an important role in the outcomes of a strategy, as in car sharing and transit improvements and incentives, the sourcebook discusses the interactions in the individual strategy reviews. Conversely, where transportation strategies may have significant effects on land use patterns, as in capacity expansion, the sourcebook notes those outcomes as well.
Association of Metropolitan Planning Organizations (2004). Noteworthy MPO Practices in Transportation-Land Use Planning Integration.
Cao, Xinyu, (2009). "Disentangling the Influence of Neighborhood Type and Self-Selection on Driving Behavior: An Application of Sample Selection Model," Transportation, Vol. 36, No. 2, pp. 207-222.
Committee for the Study on the Relationships Among Development Patterns, Vehicle Miles Traveled, and Energy Consumption, (2009). Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions, Washington, DC: Transportation Research Board, TRB Special Report 298.
Robert Cervero (2002). "Built environments and mode choice: toward a normative framework," Transportation Research Part D7, 265-284.
Ewing, R., Bartholomew, K., Winkelman, S., Walters, J., and Chen, D. (2008). Growing cooler. Washington, DC: Urban Land Institute.
Litman, Todd. Land Use Impacts on Transport How Land Use Factors Affect Travel Behavior. July 9, 2010. Victoria Transport Policy Institute.