If you’ve ever been skeptical about whether carpool policies actually work, Indonesia would like to have a word with you. Jakarta, one of the biggest metropolitan areas on Earth, had a carpool policy that seemed to be particularly susceptible to abuse. Abruptly, in March 2016, the Jakarta government announced that the policy would end in a week.

This gave a group of economists at Harvard and MIT just enough time to collect traffic data before the policy ended and compare it to the aftermath. What they found wasn’t pretty: that unpopular carpool policy was making a big difference to traffic, which got even nastier after the policy ended.

But the carpool lane is empty!

Traffic congestion is not only a rage-inducing black hole for time; it’s also terrible for climate change and air quality. Some of the worst hotspots for traffic congestion are in the developing world, where there is limited public transit and where rapid growth has happened in the era of car-centric design. Jakarta, which has a population of more than 30 million, has some of the worst traffic in the world.