In the United States, the transportation sector is the largest single source of greenhouse gases. Despite a growing trend toward vehicle electrification, the situation is hardly improving. While emissions from power plants declined dramatically over the past two decades, transportation emissions have remained flat.

The primary culprit? Automobiles, which pollute more than other modes of transportation and account for the vast majority of miles travelled in the US. Auto emissions not only contribute to global climate change but also have been linked to ill health and at least 100,000 annual deaths nationwide because of particulate pollution.

Recent federal efforts have sought to curb automobile-caused air pollution by decreasing vehicle-miles traveled (VMT) through the use of transportation control measures (TCM), projects designed to shift people out of cars and reduce travel. The Clean Air Act requires metropolitan areas with high air pollution to focus investments on TCMs (PDF) before spending on other types of projects. TCMs have almost always come in the form of direct investments in transportation projects, such as transit lines or bikeways, but other avenues for progress exist.

Two decades ago, the Environmental Protection Agency developed guidance to apply the TCM concept to land-use changes, such as building dense multifamily housing in mixed-use neighborhoods. The theory was that such investments would replace more-polluting development projects in sprawling areas. Yet, apart from Atlanta’s large Atlantic Station (PDF) project, whose infrastructure was funded as a TCM, no other major federal transportation grants have specifically focused on land-use change as a mechanism to address transportation pollution.

Overall, peer-reviewed scholarship shows that greater residential and job densities, combined with strong access to destinations, are associated with less automobile travel and less pollution. Densifying the built environment can reduce transportation-related emissions and complement the shift toward public transit and vehicular electrification.

Neighborhoods with a greater density of housing and jobs have fewer transportation-related emissions

On average, households living in the densest parts of metropolitan areas—typically central cities—consistently produce lower carbon emissions than households living in suburban, exurban, and rural areas. People living in cities drive less partially because they have more nearby destinations, reducing their driving time and allowing for more alternative travel methods, such as transit and walking.

These differences also play out at the neighborhood level within cities. An examination of California households showed that communities with residential densities that are 1,000 units per square mile lower than average had 4.8 percent higher VMT and more fuel use. Residents in more-sprawling neighborhoods not only have to drive farther to accomplish their daily needs but also are less likely to drive smaller and more fuel-efficient cars. Similar outcomes are apparent even among new construction projects in these different types of neighborhoods.

Globally, compact development in urban cores is associated with substantially reduced VMT and fewer emissions, but compact development in suburban areas doesn’t have this effect. Households living in a hypothetical housing unit five miles from downtown drive 32 percent less on average than those living twice as far away.

It may be that people who want to drive less are self-selecting for certain neighborhoods where doing so is feasible. But even when controlling for self-selection, residents in high-density neighborhoods have lower VMT. This finding is generally backed by a long history of research on neighborhood self-selection, such as one paper collecting several studies showing that residents of lower-density suburban areas drive more. Another such paper found that, on average, a neighborhood with 40 percent higher densities had 9 percent lower per capita VMT.

Scholars who have modeled individual transit-oriented development projects find that, compared with other types of development, they can substantially reduce greenhouse gas emissions, cut down on particulate pollution by up to a third, and reduce household expenditures.

The effects of land-use change vary by region—and location within those regions

High-density development doesn’t affect automobile travel equally nationwide. One group of researchers found that a densely populated neighborhood in New York could reduce VMT at twice the rate of an equally densely populated neighborhood in St. Louis. Similarly, having higher job accessibility, especially by transit, reduces driving in a neighborhood, particularly for high-income workers.

In other words, increasing housing densities in central areas of cities may be most useful in terms of reducing emissions. These projects could take the form of downtown office-to-residential conversions or new apartment buildings on former parking lots. But these effects vary by region, with the densest and most transit-accessible metropolitan areas benefiting most from VMT and emissions reductions when new high-density, mixed-use projects are implemented.

Ultimately, residential density is just one factor affecting neighborhood-level VMT and emissions. Intensity of automobile travel varies based on time of day and destination. Higher household income and car ownership increase car use. Higher employment densities shift people onto transit, as long as transit options are available. Finally, neighborhoods with a mix of businesses, restaurants, and other uses and comprehensive street grids contribute to fewer VMT.

Implications for public investment

This research illustrates the benefits of federal investment in high-density, transit-accessible infrastructure. Though there are federal grants to plan for transit-oriented development, those investments total only about $10 million annually—compared with $2.5 billion annually for congestion mitigation and much more for other highway programs. The 2022 Inflation Reduction Act also allocated funding for low-pollution transportation but offered no meaningful investment for land-use change.

It's in the interest of policymakers seeking to reduce emissions to encourage environmentally friendly land uses. Climate change scholars widely recognize the benefits of high-density, transit-oriented development. The Intergovernmental Panel on Climate Change’s 2022 report projected that, compared with “business-as-usual” scenarios, concentrating growth near transit in dense, mixed-use developments could reduce emissions by up to 26 percent. Another recent study found that changes encouraging denser construction in central neighborhoods could result in an 8 to 13 percent reduction in VMT and emissions.

Instead of directing billions in federal funding toward automobile infrastructure, transportation funds could be redirected to support land-use change through TCMs that encourage high-density development rather than polluting suburban sprawl. This approach has already proven its efficacy, with the one land-use project that acted as a TCM—Atlanta’s Atlantic Station—substantially reducing car travel for both residents and workers compared with people on average regionwide (PDF). Ideally, future investments should be paired with support for affordable housing and public transit, which can give more people the opportunity to live in low-polluting neighborhoods.