In January and April 2016, the government of Delhi piloted an “odd-even” traffic rule which mandated that only cars with odd (even) numbered license plates could ply on odd (even) dates. We use high frequency measures from air quality monitoring stations to estimate the program impact. Relative to surrounding satellite cities, fine particle concentrations in Delhi’s air were lower by 14-16% during the January pilot. In contrast, the program did not affect Delhi’s air quality during the warmer month of April. Taken together, this suggests that the main value of an “odd-even” program is as an emergency measure during winter months when car emissions play a more prominent role in affecting air quality.