Evidence shows car-pooling cuts traffic jams

In the toolbox of policy fixes to alleviate traffic congestion, road rules that promote car-pooling are among the least preferred options of politicians, but hard evidence collected in one of the world’s busiest metropolises shows just how effective the approach is.

Ironically the evidence comes from a research opportunity presented by the abandonment of a car-pooling policy, when Indonesia’s capital city, Jakarta, dropped a rule requiring cars in its central business district to carry a minimum of three passengers during rush hour.

Jakarta, with more than 10 million people, has legendary traffic jams, but congestion is a common problem even in cities with just a tenth of the population. Around the world many urban commuters can spend an extra 60 to 90 minutes hours a day travelling to and from work due to traffic hold-ups.

Several strategies have been employed to help resolve this issue, the most “popular” by far being building new roads, followed at a distance by implementing tolls and imposing license plate restrictions for vehicle use. Enforcing high occupancy vehicles (HOV) or carpooling remains one of the most disliked and contested solutions enacted by governments.

Thus the study in Jakarta, led by Rema Hanna of Harvard University’s Kennedy School of Government and published in Science, is particularly interesting since it provides evidence that increasing the number of HOVs on the road is a highly effective way to tackle traffic congestion.

Hanna and her team examined traffic-speed data from Jakarta from a week prior to the abolishment of its three-passenger policy, in late March 2016, to a month afterwards. Using data from Google Maps APIs for major roads in Jakarta, they measured travel delays – calculated from the time needed to travel one kilometre compared to the free-flow speed of the road.

Through their analysis, the researchers found that travel delays on the roads that had previously included HOV restrictions rose by 50% in the mornings, almost doubled in the evenings and even increased in off-peak times and on alternate roads by more than 10%.

This work aligns with studies on other traffic management policies like London’s toll roads and re-raises the important, yet unanswered question of how increased traffic on the roads will be managed in the future.