Air quality near the normally busy highway improved by 83 percent that day last July, relative to comparable weekends. Elsewhere in West Los Angeles, the improvement was equally dramatic. Air quality improved by 75 percent on that side of the city and in Santa Monica, and by 25 percent throughout the entire region, as a measure of the drop in ultrafine particulate matter associated with tailpipe emissions.

“We saw what we expected: you take motor vehicles away, the air gets really, really clean,” Paulson says, “which tells us that most of the pollution is from motor vehicles from one type or another in this area.”