The Obama administration envisions a “low-carbon, low-petroleum” future where Americans tele-work, tele-shop, walk, bike and use carpools or mass transit if they must leave the neighborhood at all. A study released Friday says the U.S. has the potential to reduce petroleum use and pollution in the transportation sector by more than 80 percent by 2050. In other words, gasoline-powered cars may go the way of the dinosaur, and many Americans may end up living in planned, mixed-use, “walkable” neighborhoods, built along mass transit lines. … Higher densities (more people living in the same area in smaller homes), a mix of...