Today, we can start to imagine what the future of transportation might look like. More and more of us are regularly calling an Uber or a Lyft to get to work or to the train station. In the next decade or two, many may decide it isn’t necessary to own a car at all and instead use a hand-held device to summon a self-driving vehicle when one is needed. It sounds convenient and possibly cheaper. Is it also more environmentally sustainable? Uber CEO Travis Kalanick says that Uber is already having a positive environmental impact, with fewer cars on the road and more of those cars being low-emission vehicles. But the real question is whether ride services will reduce the total number of vehicle miles that each of us travels. Early studies are mixed. One suggested that having access to a ride service makes people more likely to use public transportation, because they are confident that they won’t get stuck if they work late. Another found that a small but significant group of Uber riders would have otherwise stayed home, suggesting that the services could actually lead to more travel.