People walk on the Champs-Élysées on the day that central Paris went car-free. Philippe Wojazer / Reuters So successful was Paris’s first “day without cars” on September 27 that the city’s mayor wants to expand the effort to help clean up the city’s terrible air quality. The single-day initiative made 30 percent of the city’s roads off-limits to automobiles, but even that minor reduction in traffic was enough to cut pollution levels—and noise—significantly.