Approximately 40,000 people will die on U.S. roads this year, and thousands more will be injured. A disproportionate number of those traffic injuries will befall people from lower-income communities. According to new research, pedestrians in the poorest neighborhoods of Montreal were six times more likely to suffer traffic injuries than pedestrians in the wealthiest neighborhoods. Bicyclists and motorists in poorer neighborhoods were also at greater risk; they were four times more likely to be injured on the road.

Researchers have proposed many explanations for social inequalities in traffic injuries, including differences in the prevalence of drunk driving, use of helmets and safety restraints, and driving speeds. The new study, published in the American Journal of Public Health, finds that the street environment may be largely to blame.

"People don't think of traffic safety as an environmental justice issue," says Reid Ewing, a city-planning professor from the University of Utah who was not involved in the research. "Low-income people are disadvantaged in a lot of different ways, including traffic safety."

Lucie Laflamme, an injury epidemiologist at the Karolinska Institute in Stockholm, called the study design brilliant and said in an e-mail that it demonstrates that inequalities in road traffic injuries could be addressed by making environmental changes, not just behavioral ones.

The research, led by Patrick Morency, director of Montreal's Department of Public Health, used ambulance records to study the distribution of injuries across the city's road intersections. They analyzed nearly five years of data, including 19,500 reported injuries, and then mapped those mishaps onto the road network.

Morency's team found that lower-income neighborhoods had twice as many intersections of major thoroughfares, which tend to carry high volumes of traffic at high speeds. Even after controlling for traffic volumes these intersections had, on average, 2.4 times more pedestrian injuries than did intersections of minor roads. Using a multivariate analysis, Morency's team calculated that if the intersections of major roads were replaced with those of minor streets, pedestrian injuries would decrease by 58 percent; harm to cyclists and motorists would decline by 24 and 72 percent, respectively.

Similarly, poor communities contained a higher proportion of four-way intersections than did wealthier neighborhoods. Four-way intersections were responsible for 3.5 times more pedestrian injuries and 4.7 more motorist trauma than were three-way intersections. If poor communities contained the same number of four-way intersections as wealthier communities, pedestrian injuries would decline by 71 percent for pedestrians, 58 percent for cyclists and 79 percent for motorists.

These results are part of a growing body of literature that shows road design has an impact on safety. For example, features such as narrow street widths, the presence of sidewalks, and buildings and trees that are positioned nearer to roads can subtly influence people to drive more safely. This is the first study that has linked those features to social disparities in traffic injuries.

Morency said that there are probably two reasons why poorer communities are more likely to have less safe road designs: "It was easier to build expressways in the poorer areas because people didn't mobilize—and the land was cheaper," he says. "Once these designs are implemented, it reduces the land value, so it attracts poorer people." Similar processes are likely to be at work in U.S. cities as well.

Fortunately, it may be possible make these roads safer. "By demonstrating that particular features of the built environment are associated with injury, the study results present opportunities for preventive action," says Stephanie Burrows, a researcher at the University of Montreal Hospital Center, who also works at the Department of Public Health. "Several studies have shown that modification of the built environment is a widely used approach that can be highly effective in reducing motor vehicle injuries."

But Eric Dumbaugh, an urban planner at Florida Atlantic University, cautions that changing low-income neighborhoods to look like wealthy suburban neighborhoods might not help anyone. He said that wealthy neighborhoods are safer because they are designed to push traffic elsewhere, thus requiring individuals to drive instead of walking to reach most of their destinations. In communities where people are less likely to be able to afford a car, redesigned roads may just be another disadvantage.

Higher traffic volumes also explain much of the disparity in traffic injuries between rich and poor neighborhoods; in Montreal people in poorer communities have two to three times as much traffic flowing through them. In addition, lower rates of car ownership mean that poorer people are more likely to walk and ride bicycles, so their exposure to traffic collisions is higher. Morency says that governments could try to reduce traffic volumes—and thus traffic injuries—by encouraging the use of public transportation.

The next step will be to look at road design features at a higher resolution, Morency says. Are the poorer neighborhoods less likely to contain sidewalks and safe crosswalks? Are speed limits higher on roads in low-income versus wealthy neighborhoods? Such design features have been linked to safety in other studies, and are more easily changed than the road infrastructure itself.

Nevertheless, "billions of dollars are invested every year to build new roads and to rebuild old ones," Morency says. Oftentimes, the engineers who build those roads unintentionally incorporate less-safe features—such as wide lanes and shoulders that allow people to drive faster—or they fail to include features that make roads safer for bicyclists and walkers. Morency suggests that at least "the government could stop spending money in ways that only increase the problem."

When traffic-calming measures are installed, they're more likely to be located in wealthy neighborhoods, Utah's Ewing points out. "In terms of social justice, if I were a mayor, I would put more of my resources into traffic safety in lower-income neighborhoods, because they have a higher incidence of traffic accidents."

Increasing public transportation and making streets safer for pedestrians and bicyclists will not just reduce injury rates, Burrows says, but, as an added bonus, they could reduce air pollution and increase physical activity levels as well.