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.

The most immediate problem is one of space: how do you move millions of people to where they need to be using the limited space that cities have available? Cars take up a lot of room, and carpool policies are based on the idea that cars with only one person in them are literally wasting all that extra passenger space. Their goal is to provide an incentive to use space wisely by reserving resources for people whose cars are at least carrying more than one person.

In Jakarta, however, a carpool policy was undercut by the emergence of “professional passengers,” who would actually hang out upstream of the carpool-restricted areas and fill up people’s cars for a fee. These professional passengers may not be the norm, but there are other reasons to question whether carpool policies work—is the incentive actually strong enough to encourage significant numbers of people to carpool? Could the congestion in non-carpool lanes or roads end up being just as bad, while the carpool lane becomes its own waste of space? There has been a lack of good data to answer these questions.

Jakarta’s high-occupancy vehicle (HOV) policy was one of the strictest in the world: the main streets in the CBD required three people per car during the morning and evening peak times. Driving at all on these roads without passengers risked a fine.

So, when Jakarta suddenly announced that the policy was entering its last week of life, economists Rema Hanna, Gabriel Kreindler, and Benjamin Olken hopped to it. Two days after the announcement, they started collecting anonymized Google Maps data on driving speed from several main roads in the city. This gave them data from the last few days of the HOV policy, which they compared to data collected after the policy ended.

Almost as slow as walking

“The data from before the policy was lifted reveal that traffic was clearly bad,” write Hanna and her colleagues. It was taking between 2.2 and 3.2 minutes to drive a kilometer on three of the main roads (that's about 11 to 17 miles per hour), and it was as bad as 4.4 minutes per kilometer at some points. For comparison, cyclists in Copenhagen average about 3.5 min/km. Congestion before the policy ended was bad enough that these drivers were going slower than the average casual urban cyclist could.

The data from the few days before the policy lifted already held some clues that the HOV rule was doing something, however. On non-HOV roads, peak times were horrendous, but things got better off-peak. On the HOV roads, the peak times were actually slightly better than off-peak times.

In the first month after the policy ended, things got dramatically worse on both HOV and non-HOV roads. The results showed “a decline in average morning rush hour speeds from 28 to 19 km/hour... and a decline in evening rush hour speeds from 21 to 11 km/hour,” write Hanna and her colleagues. By comparison, they add, “typical walking speeds are about 5 km/hour.”

There are a few different reasons why this slowdown might have happened. It could have been that people were choosing to travel outside peak times or on other roads to get around the HOV policy before switching to peak times and main roads once the policy had lifted—but there was more congestion in off-peak hours and other roads, too. That suggested an overall increase in traffic volume.

The best explanation for the data, write the researchers, is that there were people who hadn’t been using their cars at peak times, but were now choosing to do so. And once they had their cars with them, they were making other trips throughout the day. It’s also possible that the congestion on some roads had gotten so bad that people were forced to choose alternative routes, spreading the congestion throughout the city. And the added delays persisted over time, meaning that people didn’t make other plans even after it was obvious that the traffic had gotten apocalyptic.

Hanna and her colleagues reach a clear conclusion: Jakarta’s HOV policy was helping, despite the professional passengers making it look like a big sham. But obviously, it’ll be necessary to look at data from other places to see if the effect transfers elsewhere—mileage may vary for different cities.

Science, 2016. DOI: 10.1126/science.aan2747 (About DOIs).