While cities like Bangkok and Mexico City face endless challenges due to the overcrowding of cars on their streets, MIT economists have found that the solution to this problem could be as simple as designing policies that would encourage commuters to carpool instead. They write their reasons in “Citywide effects of high-occupancy vehicle restrictions: Evidence from ‘three-in-one’ in Jakarta,” an article published in the journal Science earlier this month.

The authors conclude that high-occupancy vehicle (HOV) lanes, which require vehicles to have at least three passengers to be eligible to take them, are critical for busy roads. Even though HOV’s were installed on the streets of Jakarta since 1992 to tackle the city’s heavy traffic, the Indonesian capital removed them in March 2016 first for a week, then for a month, and then permanently, after the lanes drew criticism from locals. Part of this criticism were complaints about drivers giving money to children or mothers carrying children for a ride to reach the three-person requirement to be eligible to take the HOV lane, dodging traffic. This phenomenon made other commuters question the effectiveness of the HOV system and whether it really reduces the number of vehicles.



With the help of the built-in traffic-tracking technology Google Maps provides, the economists took advantage of Jakarta’s decision and studied its aftermath. They found that travel delays in the city became 46% worse during the morning rush hour and 87% worse during the evening rush hour as opposed to the years when the HOV lanes existed. The average speed of rush hour traffic declined from about 17 to 12 miles per hour (27.4 to 19.3 kilometer per hour) in the mornings, and from about 13 to 7 miles per hour (20.9 to 11.2 kilometers per hour) in the evenings.

“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,”says Ben Olken, a professor of economics at MIT and co-author of the paper.

Questioning HOV lanes is not only confined to Jakartans. In California, HOV lanes carry an average of 2,518 commuters per hour during rush hours. Like Jakarta’s municipality, California have also been questioning the actual effectiveness of HOV lanes. One argument says that HOV lanes do little to relieve congestion, since they force single occupant vehicles (SOVs) to jam together in the mixed-flow lanes, while the HOV lane next to them remain largely underused. This phenomenon which they call “empty lane syndrome” has led to the conclusion that cancelling the HOV would alleviate congestion by making better use of the excess capacity.