Freeway pollution: How we reported the story

animation of finding Census blocks within 1,000 feet of a freeway

On Thursday we published a report detailing the home-building near high-pollution zones next to freeways. This is how we reported the story.

How we measured housing growth

Under the California Public Records Act, The Times obtained a database of building permits from the City of Los Angeles dating to 2005, the year state air quality regulators began warning against building near freeways.

We counted the number of new housing units in the database that were within 1,000 feet of a freeway. The city began flagging developments at that distance as a concern in 2012 based on a growing body of science linking roadway pollution to health problems.


Download building permit data: la-city-permits.xlsx

How we counted people

We analyzed U.S. Census Bureau data to count the number of people living within 500 feet and within 1,000 feet of a Southern California freeway. We used census blocks, the smallest geographical unit available, to compare the population in 2000 with 2010, the most recent years with that level of detail.

The 500-foot zone was included because it is where the California Air Resources Board advises cities to avoid siting new homes, schools, child care centers, medical facilities and other “sensitive land uses.”

We built a map of the high-pollution zones near freeways using data from local and regional planning agencies and satellite imagery.

We added census blocks to that map, then estimated the number of people within 500 feet and within 1,000 feet of a freeway. For blocks that were partially outside those zones, we counted people in proportion with the area of the block inside the zone. For instance, a census block with 100 people that was 50% inside the freeway zone would be counted as 50 people.

Download the GIS files:

How we took air quality readings


Using two portable air quality sensors borrowed from USC, we took measurements of tiny pollutants known as ultrafine particles that are emitted from vehicles.

Over a 20-minute period, one reporter measured the air near the freeway while another took readings 1,500 to 1,800 feet downwind. We also tracked wind speed, wind direction, humidity and temperature on-site. The results were reviewed by scientists at the South Coast Air Quality Management District and USC.

Download the air quality measurements: pollution-testing.xlsx

Questions? Feedback? Contact the reporters.


Tony Barboza

Jon Schleuss