Researchers said 60 million people live within about one-third of a mile from a busy road.
"It's a surprisingly large proportion of the population," Gregory Rowangould, a professor of engineering at the University of New Mexico, told the Los Angeles Times.
Rowangould used U.S. Census and Department of Transportation traffic data to conduct the nationwide study.
The study counted people near roads where more than 25,000 vehicles travel each day, a traffic density Rowangould said he considers "the lower limit of roadways with health impacts."
People living near high-volume roadways face greater risk of respiratory illness, including asthma and lung cancer, the study said, while most U.S. counties do not have monitors to measure air pollution levels adjacent to roadways.
"If there's no monitor, a potential violation is not going to be detected, and there's not going to be the additional regulations that can help clean up air pollution in that area," Rowangould told the Times.
The study findings were similar for all areas of the country, he said, suggesting traffic pollution is not just an urban issue but "potentially a much larger and more widespread public health concern."