New Algorithm Could Save Cities from Too Many Taxis

The team used this graph to feed the algorithm and thus determine the best solution for rideshares and taxis.

"We started looking into this problem motivated by the increasing trends toward shared mobility, which will likely become even stronger with the transition to autonomous vehicles," said Ratti, who is also a professor of the practice in MIT's Department of Urban Studies and Planning. "If demand for mobility is served by fleets of shared vehicles, a fundamental question is: How many vehicles do we need to serve the mobility needs of, say, a city such as New York?"

Previous attempts to solve these issues were also found in other mathematical attempts. The most famous way previously used to name the problem was the "traveling salesman problem," in which mathematicians would minimize the total distance a salesman would travel if he had to visit a certain number of stops in a day of sales. 

"If demand for mobility is served by fleets of shared vehicles, a fundamental question is: How many vehicles do we need to serve the mobility needs of, say, a city such as New York?"

However, researchers have come across issues in finding a consistently optimal solution -- even leveraging computers and updated computing systems. There's also the issue of various companies' fleets determining optimal solutions for their particular companies. A number of factors have led to these previous attempts at calculating taxi density on the road failing, according to Paolo Santi. Santi serves as a research scientist for the Senseable City Lab and also as the Italian National Research Council CNR. That group was responsible for leading the research team. 

The researchers used data from one of the globe's most congested areas -- Manhattan. They estimated travel times using Manhattan's road network and GPS-based estimates from taxi data available to them. The team discovered that they could reduce the overall fleet size needed by 30 percent and still maintain optimal service. 

"If we were to consider replacing the current taxi system in New York with an optimized fleet of vehicles, we would have to find the best way of serving the around 500,000 trips made in a day, which are currently served by about 13,500 taxis," said Santi.

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The best part? The solution assumes that individuals will not share taxis. There are no attempts at cramming people into cars to forcibly carpool. 

"If we look at Manhattan as a whole, we could theoretically satisfy its mobility demand with approximately 140,000 vehicles -- around half of today's number," he says. "This shows that tomorrow's urban problems regarding mobility can be tackled not necessarily with more physical infrastructure but with more intelligence, or in other words: with more silicon and less asphalt."