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May 12, 2021

A Roundabout Way to Cleaner Air

By Gail Poulton
When Professor Cindy Hauser’s students counted cars in Davidson, N.C., for a study on air pollution, she didn’t believe their results.
The students had monitored traffic at one of the busiest intersections in Davidson, and also in a residential neighborhood in front of an elementary school. “After they counted traffic, they came back with absolutely phenomenal numbers for a side street,” says Hauser, an assistant professor of chemistry at Davidson College.
But the students’ numbers held up. While school enrollment is only 1,100, it generates traffic from as many as 500 cars during morning drop-off and afternoon pick-up. “So the high numbers weren’t as sustained as those at the intersection of Concord and Main, but they were very high at these time periods,” Hauser says.  
That convinced Hauser that an air quality study in Davidson was both feasible and necessary. The impetus behind the study came from the early death from lung cancer of a woman who had worked as a crossing guard at an elementary school. “She was a non-smoker so we wondered whether her job had contributed to her condition,” Hauser explains.
So Hauser’s students wired little plastic tubes filled with special filters that measure air pollution to trees and telephone polls at selected areas in the town. These collectors detected levels of ozone and nitrogen oxide – a product of burning fuel and a component of ozone.
Their conclusion? There was no significant difference between pollution exposure in front of the school and at the busiest intersection in the town of just over 7,000 people. So even though Davidson is considered a semi-rural area, the students found urban-sized air pollution.
Keep Traffic Moving to Cut Air Pollution
The problem isn’t just the number of vehicles. It’s that the vehicles are stopping, starting, and idling. These actions all produce more emissions than simply driving down the road.
It’s not a long leap, then, to understanding that if you can keep traffic moving, you’ll cut down on emissions – and air pollution. That reasoning can serve as a rationale for replacing traffic lights with roundabouts – circular intersections designed to keep traffic flowing.
“There are good reasons to have a roundabout in terms of air quality,” says Hauser, who will be working with students on another type of air monitoring project this summer that focuses more on ozone. 
Roundabouts provide a way for cars to get from one street to another without stopping at a traffic light or stop sign. When cars stop and go for short periods, it causes more air pollution than if cars keep moving. Plus, starting and stopping uses far more gasoline than cruising.
“You can design traffic signals to give more green time, but it delays the side streets. A roundabout removes that. As long as no one’s coming, you’re free to go,” explains Pate Butler, regional traffic engineer for North Carolina Department of Transportation (NCDOT) District 10, which covers five counties including Mecklenburg where Davidson is. “They’re especially good on a lower volume road where you sit at a signal even when no one is coming,” Butler says.
But they can also keep traffic moving on major roads, she adds. In fact, NCDOT constructed roundabouts in Davidson on two main roads between Interstate 77 and downtown in 2008.
“The traffic wasn’t alarming at the time we began planning them, but looking at the projected volumes, you knew it would be a huge problem later on,” says Lauren Blackburn, the town’s planning manager. “We were concerned about air quality, as well as minimizing congestion … It’s not a good thing for us to have idling traffic when we’re already out of compliance as far as air quality.”
“You can even have dual-lane roundabouts that handle a lot more traffic,” explains Butler. Roundabouts generally are designed to handle trucks, buses, and emergency vehicles, too, and can help them get more quickly through traffic and to their destinations, according to Butler.  
Roundabouts Carry a Host of Benefits
Generally, roundabouts are constructed to improve traffic flow and safety. One study concluded roundabouts reduce collisions by 39 percent and collisions with injury by 76-78 percent, mainly because cars in roundabouts travel at slower speeds. Also, crashes that occur at roundabouts tend to be “fender-benders” rather than the serious T-bone, left-turn, or head-on collisions that frequently occur at traditional intersections.
In North Carolina, which built its first roundabout in 1999, crashes of all kinds have been cut almost in half where roundabouts have been installed at intersections, according to a 2010 NCDOT publication, “Your Guide to Understanding Roundabouts.” (www.ncdot.org/doh/PRECONSTRUCT/traffic/TEPPL/Topics/R-38/R38_br.pdf)
But studies show roundabouts can also save gas and decrease air pollution, according to James Dunlop, NCDOT congestion management engineer. “Emissions and fuel consumption are becoming more important considerations,” he notes. “Roundabouts keep vehicles moving. They slow down but they don’t stop and then start again. The worst thing for emissions is to stop and then start again.”
Studies of roundabouts’ effect on air pollution show they can lower fuel use from eight percent to as much as 30 percent. According to one study in Vermont, installing a modest number of roundabouts throughout the United States would save twice as much gasoline as additional drilling for offshore oil would produce.
In addition to saving money, reducing fuel consumption reduces emissions and improves air quality. That can have a positive affect on health. Breathing ground-level ozone can worsen asthma, bronchitis and emphysema. It has also been linked to lung cancer, heart attacks, Hodgkin’s disease and premature death – especially in people with heart and lung disease.

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