Cities plagued with terrible traffic may be overlooking a simple, low-cost solution: high-occupancy-vehicle (HOV) policies can reduce traffic drastically, according to a new study coauthored by MIT economists.
In Jakarta, Indonesia, travel delays became 46 percent worse during the morning rush hour and 87 percent worse during the evening rush hour after a policy requiring three or more individuals in a car was discontinued on important city-center roads. “Eliminating high-occupancy-vehicle restrictions led to substantially worse traffic,” says economics professor Ben Olken, who coauthored a Science paper on the study with doctoral candidate Gabriel Kreindler and the Harvard Kennedy School’s Rema Hanna. “That’s not shocking, but the magnitudes are just enormous.”
The researchers also found a spillover effect: traffic suddenly became significantly worse on surrounding roads. “HOV policies on central roads were making traffic everywhere better, both during the middle of the day and on these other roads during rush hour,” Olken says. “That, I think, is a really striking result.”
Jakarta instituted HOV regulations in 1992 to address its notoriously bad traffic, requiring three people in each vehicle on some major roads between 7 and 10 a.m. and between 4:30 and 7 p.m. However, many commuters picked up so-called jockeys—people willing to ride in their cars for a small fee—instead of carpooling. Skeptics contended that the policy therefore wasn’t actually reducing the number of vehicles on the roads. So Jakarta scrapped it in 2016—first for a week, then for a month, and then permanently.
When the government announced that it would suspend the policy, the researchers saw an ideal opportunity to conduct a natural experiment. Within 48 hours, they began querying Google Maps data to track traffic speeds on several Jakarta roads every 10 minutes; they captured 2.5 weekdays’ worth of data before the rules were lifted, and then eight weeks of data afterward.
After the HOV policy was abandoned, the average speed of Jakarta’s rush-hour traffic declined from about 17 to 12 miles per hour in the mornings, and from about 13 to seven miles per hour in the evenings. By comparison, people usually walk at around three miles per hour.
The researchers note that results might vary elsewhere, depending on, say, a city’s layout or public transit system. But implementing HOV policies, whether on entire streets or in certain lanes, is inexpensive. All a city really needs are signs, paint for markings, and enforcement. “I don’t think we should necessarily take the result and wildly apply it everywhere, but [given] the kind of really serious congestion problems Jakarta has, it suggests this is a policy measure that has the potential to work,” Olken says.