We’re all exposed to unhealthy traffic pollutants, but people who spend a lot of time on or very near higher-traffic roads get more. The Center for Public Integrity and Reveal from The Center for Investigative Reporting teamed up to look at the schools across the country that sit within 500 feet of busy roads.
We picked that distance because, in general, studies suggest that the biggest daytime exposures are within the first 500 feet from the road (though some studies have found elevated levels farther out, such as roughly 900 to 1,000 feet). California’s school-siting law, which aims to keep new schools away from freeways and other major routes, uses 500 feet as the area of concern.
The California law focuses on very heavily traveled roads, but there’s no true dividing line between bad and OK. Some studies have found health effects among people near roads with at least 10,000 vehicles a day, which includes routes with a tiny fraction of the traffic on an L.A. freeway. In fact, because steady speeds produce less pollution than acceleration, vehicles on highways that aren’t plagued by stop-by-go congestion are cleaner than they are on lower-speed roads with traffic lights and stop signs. And a road that draws diesel trucks, particularly old trucks, could be worse than a higher-traffic route with only cars.
We tried to account for these complexities with our traffic thresholds. We ended up defining a “busy road” as one with average daily traffic of at least 30,000 vehicles, or 500 or more trucks and at least 10,000 total vehicles.
We used schools data tracked by the National Center for Education Statistics, part of the U.S. Department of Education. It includes latitude and longitude for every school, along with information ranging from the type of school to the demographic details on the student body. The most recent full dataset from the NCES is for the 2014-15 school year.