Researchers at the Massachusetts Institute of Technology (MIT) set out to answer that question, analyzing a year's worth of New York taxi trips. They concluded that the total amount of time that taxis spent traveling could have been reduced 32 percent by shared cabs, with passengers riding no more than an extra five minutes. That reduction in travel time would result in less pollution and traffic, the researchers said, though they did not specify how much. Their work was published Tuesday in the Proceedings of the National Academy of Sciences.

The researchers also looked at whether such increased efficiency would be possible in other cities that have a lower density of taxis than New York. "You still find that there's