Sept. 2, 2014

Math reveals benefits of taxi sharing

Steve Strogatz

It’s true that most people would sooner ride a cab alone or with friends than with a stranger. But ride sharing could have a powerful impact on the economy and environment – benefits quantified for the first time by mathematical modeling.

Steven Strogatz, the Jacob Gould Schurman Professor of Applied Mathematics in Cornell’s College of Arts and Sciences, collaborated with Massachusetts Institute of Technology researchers to reveal the vast untapped potential of New York City’s taxi system, with results that could be applied to other urban areas. The study, led by Carlo Ratti of the MIT Senseable City Lab, was published online in Proceedings of the National Academy of Sciences (PNAS) Sept. 1. The analysis used data from the New York City Taxi and Limousine Commission, which counted 150 million taxi cab rides in 2011.

The mathematical tool the researchers used to draw optimal sharing routes was called “shareability networks.” The key idea was to express the shareable resource, in this case, a taxi trip, as a node in a network, and to draw links between two nodes if the corresponding resources – trips – could be shared.

Analysis revealed that in theory, based on 2011 numbers, the total number of taxi rides in New York could be reduced by 40 percent, and the operational costs by 30 percent, with only minimal delays in bringing passengers to their destinations – a few minutes at most.

As for the environmental impact in traffic and emissions: “Imagine New York with calmer streets and clearer skies,” said Paolo Santi of the MIT team.

Of course, the researchers say, there’s the obvious problem of inconvenience for riders and the discomfort of sharing with strangers.

“Although we haven’t managed to solve the stranger problem, we have solved the time problem,” Strogatz said. “Using network theory, we figured out an efficient way of matching potential riders, based on where they are, where they want to go and where the cabs are. The predicted economic and environmental savings are considerable.”

To demo their analysis, the MIT Senseable City Lab has developed an online tool call HubCab to visualize taxi trip computation in New York.

The study, “Quantifying the Benefits of Vehicle Pooling With Shareability Networks,” was sponsored by the ENEL Foundation, Audi Electronics Research Laboratory, General Electric and the MIT Senseable City Consortium.