It’s not in your head. Traffic congestion really is getting worse. According to a 2020 report from the US Department of Transportation, the average person living in 1982 in one of the 75 largest cities in U.S. faced seven hours of travel delay per year. By 2001, that figure jumped up to 26 hours of delay per year.

Traffic congestion is more than just an annoying time suck. It can have negative impacts on health due to the air pollution emitted from cars idling, with one study estimating that the emissions from passenger vehicles spike by as much as 200 percent during rush hours which raises the concentration of air pollutants in areas surrounding highways. It affects a lot of people, with more than 11 million Americans estimated to live within 150 meters of highways.

[Related: Why congestion pricing reduces traffic better than new highway lanes.]

“Traffic delay has steadily increased across the United States since the 1980’s, yet we know very little about the unique influence that traffic delay may have on population health,” Mary Willis an assistant professor in epidemiology at Boston University told PopSci.

Willis is the lead author on a study published today in Science Advances that examines the role that traffic congestion could play in a specific health scenario: birthweight. “While there are many health outcomes that we could have examined, low birthweight is a particularly important one as it can lead to immediate consequences, like trouble breathing, and long-term issues across the life course, like cardiovascular disease, cognitive impacts, and premature mortality,” said Willis. For the first time, the team was able to draw a link between traffic congestion and pregnancy outcomes in nearby neighborhoods in the U.S.

Willis and her colleagues examined the relationship between traffic congestion and the outcomes of 579,122 births in addresses within 500 meters of road segments in Texas, from 2015 to 2016. In this data, they found a strong correlation between traffic congestion and lower birth weights, with birth weights on average 9 grams lower in the highest quintile of exposure to traffic delay. The study notes that more research is needed to determine whether other socioeconomic influences (nutrition, income, access to prenatal care, etc.) not included in this research have any affect on this association between air pollution and adverse birth outcome. These results show a correlation between air pollution and low birthweight, but not that the pollution is the direct cause, since other forces could also be at play.

“A 9-gram decrease alone isn’t a clinically significant result, but this result indicates that some sort of biological impacts may be happening, which will push some babies into a clinically relevant adverse impact,” Willis said. “When you multiply that by 27 percent of all births being in high congestion areas, that small decrease in birthweight does translate into a substantial potential impact at the population level, affecting up to 1.3 million babies per year.”

According to Willis, one of the surprising results of this study is a whole new way to measure exposure that was separate from tailpipe emissions from a vehicle. Instead, this metric focused on the process of delay on the road. “Traffic engineers and urban planners can parse this data to figure out where vehicle speed is slower than expected, which indicates traffic congestion and delay. In our analysis, we used the aggregated connected vehicle and device data to examine ‘traffic delay,’ the total person-hours of delay on the roadways near maternal residences,” Willis explained.

[Related: Pollution kills 1 in 6 people worldwide.]

Tailpipe emissions are largely regulated by the federal government. However, solving traffic congestion relies on much more local policy changes. While this study doesn’t look at how to do that, some studies show that small steps such as putting up sound barriers or vegetation barriers and updating zoning laws to keep schools or daycares from being built near highways can help.

One solution that won’t help is building bigger highways. In an interview with PopSci in August, Nicholas Klein, an assistant professor of city and regional planning at Cornell University said, “For many decades in the US, people foolishly tried to solve this problem adding more lanes,” he explains. “It does not work because of induced demand. Any time you add capacity, it causes people to change their behavior.” Klein says that people will change their behavior by deciding to drive rather than taking public transportation if they see more lanes available.

Exposure to traffic is also not distributed equally. “We’re in the process of diving deeper into the socioeconomic and racial disparities in exposures to traffic congestion,” said Willis. “In particular, we’re interested in considering how patterns of disparities may have changed over time as other processes like urbanization and gentrification happened in metropolitan areas.”

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