In January and April 2016, the government of Delhi piloted the ‘Odd-Even’ traffic rule, which mandated that only cars with odd numbered license plates could drive on odd dates, and even numbered plates on even dates. The pilot lasted for a fortnight during each round. EPIC-India researchers used data from regulatory air quality monitoring stations to estimate the program’s impact and contribute to the discourse around the program with rigorous analysis. By comparing pollution measurements in Delhi and surrounding satellite cities before, during and after the program, we found that fine particle concentrations in Delhi’s air were lower by 13% during the hours the program was in effect in the January pilot; no effect was seen at night when the program was not in force. In contrast, the program did not affect Delhi’s air quality during the warmer month of April. This is possibly due to faster dispersion of pollutants in the atmosphere when temperatures are higher, effectively wiping out local differences.

Taken together, this suggests that the main value of a rationing program like the Odd-Even in Delhi is as an emergency measure during winter months when car emissions play a more prominent role in affecting air quality.