There’s a popular meme that circulates among the Internet’s urbanists every few months. Taken from above a city street, the photo shows a small crowd of pedestrians alongside a bus. Behind them is a pack of cyclists. Stretching into the background is a long line of cars.
The image and its many variations illustrate how different modes of movement require vastly different amounts of space. A block-long traffic jam has the human population of a bus, or a busy crosswalk.
That depends, of course, on a fact of car use in the developed world: Cars fit five, but people mostly drive alone. In 2009, according to the American Community Survey, nearly nine in 10 U.S. car commuters drove to work by themselves.
How much space and time could be saved if urbanites were willing and able to share rides? How much traffic and pollution avoided?
A groundbreaking study from the Massachusetts Institute of Technology goes some way toward answering those questions. The researchers, led by Dr. Paolo Santi, developed and applied a “shareability” algorithm to a year-long record of New York City taxi rides.
The average cab ride, they found, could be shared with any of a hundred other riders.
If New Yorkers were willing to share a taxi with just one other party, and tolerate a maximum of 150 seconds of additional travel time, approximately 70 percent of NYC cab rides could be shared. Taxi travel time would drop by 25 percent. The total distance traveled by New York City cabs would fall by 40 percent, relieving traffic, reducing air pollution and speeding up travel for everyone else on the road.
To put that 40 percent figure in perspective, consider that when Paris instituted alternate driving days in March (odd-numbered plates one day, evens the next), traffic dropped by a comparatively meager 20 percent. Though New York has only 13,500 registered cabs, they are nearly always in motion. With 400,000 rides into, out of and within Manhattan per day, cabs make up a sizable share of traffic in the most congested part of the city.
Sharing a ride is common practice elsewhere in the world. In Algiers, taxi collectifs heading to popular destinations pick up passengers en route as a matter of course. In Nairobi, the colorful mutatu fleet is a fixture on the roads. Nearly every city in the world (including New York) has some form of transportation that operates as a cross between a bus and a taxi.
A more tech-savvy form of car-sharing has begun to work for long-distance trips — witness the success of Germany’s Carpooling, Canada’s Kangaride, or Britain’s Liftshare. And then there’s always the non-monetary car-share economy: carpooling and hitchhiking.
But as smartphones revolutionize the transportation industry, New York has the potential to inaugurate a rather unique taxi-share program, with rides as prompt, direct and trustworthy as in a cab — but for as little as half the price.
The research was made possible by a unique experiment the city performed in 2011. That year, 150 million cab trips started or ended in Manhattan. The city tracked the time and location of every pickup and drop-off. You can see it visualized at the study’s website, and watch how ride-share possibilities fluctuate based on time of day and rider preferences.
A number of variables are at play here. How much time is a passenger willing to add to his or her trip? How many parties can share one cab? And how much time elapses until a taxi route stops considering possible pickups?
The researchers graph these and other variables in the study, published this week in the Proceedings of the National Academy of the Sciences. If passengers were willing to add up to five minutes to a journey, for example, approximately 90 percent of New York cab rides could be shared. Interestingly, there’s no sizable benefit to sharing a ride between three parties as opposed to two, at least with “delay times” under three minutes.
But perhaps the study’s second-most-important finding is this: Even with 100,000 trips per day — a quarter of New York’s taxi traffic — the city would reach “saturation,” or “near-max shareability.” In principle, that means that cities much smaller than New York would be able to derive similar benefits from legalizing and facilitating taxi-sharing.
On the one hand, this project has much in common with other data-driven approaches to urban life, like the mathematics of bike-share, trash collection or highway tolls. This algorithmic approach to urban management is often referred to as the “smart city,” one where every facet of urban life can be quantified, calculated and optimized. Carlo Ratti, one of the paper’s authors and the director of MIT’s Senseable City Lab, reminded me of former New York Mayor Michael Bloomberg’s favored expression: “In God we trust. Everyone else, bring data."
Such a system also represents an obvious follow-up from the smartphone-ordered taxi business pioneered by Lyft and Uber. (Indeed, both companies have offered riders the possibility of sharing vehicles with their peers.) Like ride-share services, a taxi-share puts empty space to use.
But the effect is quite different. Uber and Lyft create new markets. They put more drivers and cars on the road, and create more paying passengers. On the whole, they increase the size of the taxi industry. Rides aren’t being shared; they’re being sold.
A taxi-share system would likely do the opposite, decreasing the amount of money New Yorkers spend on cabs. New York’s taxi fleet is busy — yellow cabs are vacant only 25 percent of the time — but the projected 40 percent drop in cab mileage produced by a modest ride-share scheme could presumably render several thousand Manhattan cabs obsolete. That would be great for pedestrians, cyclists, drivers and anyone who breathes the air in New York. But what would induce the taxi industry to participate?
There are still plenty of people who consider a taxi ride a dearly bought moment of solitude in a busy city, made all the more indispensable by its contrast to urbanites’ many shared spaces (in the true sense of “something used jointly”) like parks, sidewalks and public transport. The German philosopher Georg Simmel wrote in 1900 that “the pecuniary character of relationships, either openly or concealed in a thousand forms” was a crucial protective layer in the overcrowded mass of the modern city – a way of ensuring a psychological distance to keep everyone sane.
But the sense of economic and physical territoriality that has long governed both taxi rides and personal cars may be blurring. Cab-share apps like Bandwagon.io aim to crack the single-taxi-rider inefficiency from the outside. Companies like Zipcar offer the practicalities of owning a car without the pains. The German car company Daimler recently purchased a couple of ride-sharing apps. Even the auto industry has conceded that personal car ownership will play a smaller and smaller role in the future of transportation.
All in all, these events presage a tremendous decline in the urban car population. Imagine that pedestrian-bus-cyclist-car meme with a shared fleet of bikes, each of which can be used by five different people each day. Imagine the cars piled full of travelers with common destinations.
Looking forward to a city of driverless cars with a vibrant culture of sharing, the researchers estimated (in an unpublished calculation released to the New York Times) that four out of five cars in the city would become superfluous.
Imagine cutting a city’s car population – on roads, in parking lots, garages, driveways and curbs – by 80 percent. What would we do with all that space?