For many Americans, proximity to high-traffic roads diminishes the quality of their housing. Cars and trucks produce harmful air and noise pollution that are associated with asthma, respiratory illness, cardiovascular disease, lung cancer, and increased mortality.

These ill effects are not borne equally. A long history of racist and antipoor infrastructure policy has meant that renters, households with lower incomes, and households of color are overburdened by transportation pollution. After World War II, for example, cities and states often used federal dollars to construct highways and then build public housing in the surrounding areas.

Current policy continues to perpetuate these environmental injustices. Using the city of Los Angeles as a case study, we explore the relationships between high-traffic roads—typically highways—and housing. We show that current zoning policy disproportionately focuses high-density housing, including federally subsidized buildings, close to highways compared with single-family homes.

Why does this matter? Renter households with low incomes—which are disproportionately composed of people of color—are most likely to live in multifamily buildings. Ironically, these same groups are less likely to drive on high-traffic roads despite living closer to them, meaning they may suffer the harms of traffic pollution without benefiting from increased mobility.

Even so, policymakers can ensure higher-density housing is integrated into neighborhoods free from the highest-traffic roads and worst levels of traffic pollution by addressing transportation needs through investments in multimodal infrastructure and services.

Zoning may contribute to inequitable siting of multifamily housing near high-traffic roads

Local governments hold expansive powers to regulate where new housing is built. In Los Angeles, the city government’s current zoning policy inequitably exposes residents to traffic pollution.

When measuring zoning for high-density housing (five or more units per lot) against proximity to high-traffic roads (those with 50,000 or more trips on a typical day), we find that dense housing is almost twice as likely to be located near high-traffic roads. While 20 percent of city land zoned for high-density housing is located within 300 meters of a high-traffic road, where traffic pollution is typically most concentrated, just 12 percent of lower-density-zoned land (four or fewer units per lot) is located near such roads.

These disparities could be explained in part by the location of transit stations. Increasing housing availability near transit is widely considered good planning practice. Indeed, the City of Los Angeles’s Transit Oriented Communities (TOC) program offers developers incentives—such as reduced parking requirements and increased maximum densities—in exchange for including affordable housing.

Current city policies may then have the unintended effect of zoning for new, dense housing—including affordable housing—near high-traffic roads. We find 33 percent of the city’s transit stations are located immediately adjacent to or within 300 meters of a high-traffic road, similar to national data. We also find that Los Angeles’s TOC zoning disproportionately encourages higher densities not only near transit but also near polluting roadways. Nineteen percent of all TOC-zoned land is within 300 meters of a high-traffic road, and almost a quarter of TOC zones that allow for the highest density construction—called tier 4 zones—are located close to such roads.

That said, 81 percent of TOC land is farther from major roads, and the remaining 67 percent of transit stations are not within 300 meters of a high-traffic road, demonstrating that transit and higher-density housing do not have to be built near polluting highways.

These zoning disparities have contributed to differences in building patterns, with apartments much more likely than single-family homes to be built in areas near high-traffic roads. We find that 19 percent of all new residential permits in the city of Los Angeles since 2013 were within 300 meters of a high-traffic road, with 12 percent of single-family permits near these high-exposure zones compared with 25 percent of units in the largest, 100-or-more-unit buildings. Further, 19 percent of units in recently developed, federally subsidized low-income housing tax credit projects, which are designed to be affordable for families with low incomes, were built within 300 meters of a high-traffic road.
 

Share of units by distance to nearest high-traffic road, by permitted development size

Opportunities to encourage more development farther from pollution—and to reduce exposure for residents near roadways

Fortunately, cities have multiple mechanisms available to begin reducing pollution exposure for existing housing and for working toward more just and sustainable housing and transportation ecosystems.

To reduce inequities in pollution exposure, cities can zone for multifamily housing near transit stations that are far from highways and can site new transit stations away from high-traffic roads. In some countries, development regulations prohibit new residential construction near high-traffic roads. Los Angeles’s TOC program allows higher-density housing in many areas that are both well served by transit and distanced from the highest levels of traffic pollution.

In Los Angeles—and likely most other large cities—hundreds of thousands of existing units and more in the development pipeline are already located adjacent to high-traffic roads. But city leaders have multiple opportunities to protect the people living in these units.

• Reducing automobile use. Cities can increase the amount of land appropriate for future housing development by reducing existing vehicle traffic, converting what are currently high-traffic roads into less toxic corridors. Strategies to reduce automobile use include regulatory approaches, such as pricing on-street parking relative to demand; physical modifications to roadways that narrow or eliminate vehicle traffic lanes in favor of nonautomobile modes, such as bus rapid transit and bicycling; and investments in other transportation modes, such as safe, accessible, and pleasant pedestrian infrastructure, frequent public transit service, and protected biking facilities.
• Reducing exposure to vehicle pollution. Cities can mitigate the exposure of people who live, work, learn, or play near high-traffic roads through strategies such as planted buffers to mitigate air and noise pollution, to reduce ambient temperatures, and to support stormwater management (PDF); building ventilation upgrades to improve air quality; and acoustic retrofits to reduce noise pollution.

With this broad suite of policy and fiscal tools, cities can reduce high-traffic roadways and protect new and existing housing from traffic pollution.