New Delhi may be the world’s most polluted city, but it’s making an effort to relinquish that title. With pollution from particulate matter at potentially lethal levels early last December, city officials took a drastic step: they announced that they would temporarily restrict the use of private vehicles by allowing owners to drive only on alternate days, based on the sequence of their number plates.

The initial results of that 15-day trial, which began on 1 January, are now in. Although traffic actually increased in the first week of the ban, the levels of PM2.5 — parti­culate matter measuring less than 2.5 micrometres across — fell by roughly 10%. That is a victory not just for New Delhi officials, but also for the scientists who sprang into action to collect the data necessary to determine whether the test had achieved its goal.

“This experiment with ‘live research’ has been really quite exciting,” says Santosh Harish, assistant director of the India centre of the Energy Policy Institute at the University of Chicago (EPIC-India). EPIC-India and the New Delhi-based Council on Energy, Environment and Water (CEEW), an independent think tank, used video monitors around the city to document the types and numbers of vehicles on the roads. The groups had less than a month to collect baseline data before the driving restrictions began.

But they weren’t the only researchers interested in Delhi’s living lab. Economist Gabriel Kreindler of the Massachusetts Institute of Technology in Cambridge scrambled to secure human-study approval and funding for a survey of driver behaviour during the traffic restrictions. Within 18 days of the announcement of the driving ban, he had arrived in New Delhi to oversee a surveying team from the Abdul Latif Jameel Poverty Action Lab’s office there. Kreindler’s work eventually found that the alternate-day restrictions were well received by most drivers, who, in spite of the disruption, were willing to comply and alter their behaviour for short periods of time.